I will likely have to get off and lead him across the brook because I have gone through that trail before when there was less water and had to do just that. I just want to be prepared for the possibility that he will jump it (where we'll cross, there is a reasonably deep and narrow ditch) and do not want to lose hold of the reins if he does.
I do think I'm going to work him outside the arena as often as possible for as long as possible.
I'm probably closer to your age (I'm 59) than you know and I'm real sympathetic. The post wasn't meant to be accusatory or anything like it, but to show you where you're "borrowing trouble" and how you are getting anxious before you even get to the trouble spots and to suggest a way to try to combat the Megrims so you don't get yourself so haired out mentally that you have a meltdown before the horse does. BTDT doesn't come out well if you do.
Had a thought, and I don't know if it's possible where you are. When introducing stream crossings to green horses I like to do one of 2 things.
First time, if there is room, I like to walk down to the stream and let them just stand until they paw, drink or whatever. Then I like to let them stick a foot or 2 in and encourage them to eventually just stand with their 2 feet in the water. Eventually they get all 4 feet in and then I like to turn them so that when they start to walk they're walking in the stream bed. Only if it's clear and not a treacherous bottom of course. If I can, I'll walk for a mile in the water until they are totally bored with the whole thing. Then go back to the crossing and out and on your way.
Second time, I walk down to the stream and then turn a 45 degree angle and cross at an angle that makes it way too broad for them to jump, they have to walk through. I have a mare (Patti, for those who know her, any surprise?) who will jump even with the lowest branches on the trees just for the fun of it, if I'll let her. First time she "steeple chased me" through a crick I almost came unhorsed from the surprise. I had no idea she could jump like that. So, to avoid any more rude surprises, I started approaching streams and cricks at a big angle and if she jumped she landed in the water. Rude surprise on HER, not me. She quit jumping and of course, the more cricks and streams we did, the less likely she is to get airborne now.
My last resort trick, and only if the water is still, narrow and no weird footing, is to back her through. Going backwards seems to convince them to quit being so silly rather quickly.