Hold on to your hat, this is going to be long! Haha
I have a mare with ERU as well [Equine Recurrent Uveitis is Moon Blindness' "technical" name - I just like to call it by it's technical name because it can be such a depressing disease and ERU makes me feel like I'm more in control of it, less at the whim of the disease. Haha]. She's had it for who knows how long but she went undiagnosed until she had her first "episode" with me as her owner, then it was discovered that she's good as blind in her right eye and very close to blind in her left eye. Maybe that's similar to your situation?
My girl does really well blind. I had owned her for nearly 4 years before that episode, never knew a thing! Haha
My girl, Lacey, just turned 28 so she's quite a bit older than your mare. Lacey isn't the average 28 year old though: we still regularly trail ride, we teach riding lessons to kids, you know the usual-ish stuff. No slowing down for Miss Lacey! Haha
Anyway, right after she was diagnosed, I spent as much time as I could researching blind horses, Moon Blindness, and ERU. I printed everything so I could hold it in my hand and highlight interesting/vital information.
I lovelovelove that article LouieThePalomino linked to, that's one of the best I've found. Super informative but not depressing.
It's easy to start feeling really let down after that kind of diagnoses and so many of the articles out there ARE so depressing so I love that that one has such a hopeful tone.
So yeah, step 1: Google search every term you can think of that pertains to blindness/ERU/etc. Read everything. Get educated. You're your horse's advocate even more now. So many people and vets don't know a lot about uvietis, it's important that you know what you're dealing with.
On the pasturing questions, I've found that space seems to be a real need for these horses. Sure, they can function in a smaller space and Lacey did just fine when she was in a pasture that was a single acre. However, about a year ago, I moved her to a pasture, adjacent to her old one, that's 6+ acres.
Since the move, I've seen an increase in movement and she seems to feel safer. In the old pasture, she had a single "running path" that she could maybe fit 5 canter strides into and she spent most of her time standing in one corner of the pasture, next to said running path.
In the bigger pasture, she still has "running paths" but she has a solid 4-5 of them and most of them are long enough that she's regularly able to gallop.
Since she has more paths through the pasture [at least with her, she uses specific "pathways" - she'll graze off the pathway but she seems to use the path as a means of self-orientation] she spends a lot more time moving around and exploring.
I'm not sure how much of that has to do with pasture set-up either because this 'new' pasture is basically 3 separate pastures with one connecting "strip", but she has access to all areas. In any case, it's been really successful for her.
I can't be of much help with fencing because the majority of my fencing is high-tensile wire covered in blackberry bushes. The blackberry bushes seem to help her stay off the fence because they brush her before she hits the fence-fence! In any case, that's another thing that's worked out super well.
I would look into something that she'll "bounce" off of instead of breaking, as a fencing material. I've found that if my girl runs into something and it scares her at all [like a fence breaking might], she's nervous for days about the area where the incident took place.
Companions: I have 2 goats for my girl and it's the BEST. They each have bells on their collars so she can hear them at all times and they have a separate stall area that they can go into to be away from her/stay in at night when she's locked in her stall [I don't stall them together because there have been close calls where a goat will get too close to her feet and half trip her, etc] at night during the winter [I don't let her be out at night during the winter because her pasture is hilly and I don't want her injuring herself slipping on mud when no one can see her - the neighbors all have my number and watch out for her for me when I can't]. One of the goats, the younger one, was just 6 months old when I brought him home and he's latched on to Lacey like she's his bestie for forever. He, being the young guy he is, will often "lead the charge" when they go places and Lacey will walk behind him with her head down next to his rump, letting him take her wherever. The older goat is quite a bit shier of Lacey. I think that probably works out to her benefit since two goats bouncing around her, feeling totally comfortable, might be a little overwhelming for Lacey. It's a good mix.
I do work hard to keep Lacey from getting herdbound too much on to the goats. It's great that they help her in the pasture but I need her to be focused on me when I'm taking her places, not on her buddies that we left behind. So that's something to be mindful of. Herdbound horses are no fun and blind herdbound horses can be downright dangerous.
As far as feed bins, I use a slow feed hay net with Lacey, hung at wither height - well off the ground. Her grain goes in a bucket, in the same corner feeder every time. Her stall is also her run-in, so I just feed her in there - easier than having food everywhere and I think it encourages her to use her run in/stall. She has water up by the gate and in her stall as well [basically on either side of the pasture].
A major thing is deciding where her food/water is going to go and committing to that placement. It's not great to be constantly fiddling with food placement and really not great to be fiddling with water placement. If you need to significantly move where her water is within her pasture, I've had luck showing her water in the new place and leaving her "old" water where it was. She'll begin drinking the old water down and as it gets lower, she should begin to think about other water sources = the new water. It might take a few refills of the old water but she'll get it and once that old water is empty, you should be able to remove it completely without issues.
Another huge thing that I've found super useful is wearing my keys on a carabiner that's attached to a beltloop on my jeans. The jingle of the keys helps Lacey know where I am and who I am. She actually associates that jingle sound with me and will begin searching for me if I hide from her and jingle my keys!
I know people say to talk to blind horses to let them know where you re...but I'm just not that vocal ALLLLL the time! Haha My keys do the talking for me. And I like to think that they help "save" my voice for really important stuff like "whoa" [super important to have a good whoa on a blind horse!!] etc - she knows to listen when I start actually talking.
That's another thing, WORDS!! Start thinking about what words you want to mean what and start just using them in your everyday scenarios. They don't have to be complex and there doesn't have to be a word for everything but she needs you to describe the world to her so she doesn't trip while you're leading her out of her comfort zone [it's hard for us to really imagine, but just being led off the "trails" your horse uses everyday is a HUGE exercise in trust] or while you're riding her.
Lacey and I use "ho" - stop right now and don't move until I say
"ok" - disregard all precious cues, everything is safe for you to carry on normally
"careful" - you're doing ok right now, but there could be a blockage/difficult footing in your path very shortly so take careful steps
"easy" - you're too excited for the current space/ground situation, tone it down
"STEP" - there's something you could trip over coming up, you need to lift your legs really high
"ah-ah" - you need to stop whatever you're doing right now, that's not an appropriate behavior [this will be coupled with "careful" if it's an issue of bonking into someone or something, "ah-ah" is mostly a noise that mean "you're being naughty!" on its own]
Is your girl fully blind yet or just close? If she's not completely blind, I have a supplementation regime that I've kinda perfected.
It's cheap and it works for Lacey. Each ERU horse is different though so what works for Lacey may not work for your girl.
Also, flymasks. Do you have her in a flymask with high levels of UV protection? If she's nearly blind, she needs a really high percentage UV blocking mask to wear whenever the sun is shining - higher than anything on the market except for "The Guardian". I've heard great things about The Guardian, I'm just poor. [one of these days I'm going to spring for one but that'll be after some saving]
Anyway, I can give you some pointers on making your own UV blocking mask if you're at all sewing-capable. It sounds+looks hard but it's not. And at less than $20 for a 90% UV blocking flymask...worthhhh it. :)
Feel free to PM me if you ever need to talk. I don't have all of the answers but I sure understand what you're going through! That was me about 2 years ago.
ETA: well played, RoperChick! Well played.