Sounds like you have a good trainer behind you, which will definitely help with your success.
My mare "got it" within about 3-4 months, but it took her another 6 months to be able to hold it more than a few strides at a time, and even longer for her to hold it for a whole ride (middle of the next year?). She's been a pasture pet and weekend trail horse for the last 2 years, so it will be interesting to see how long it takes to get her back in shape, lol.
ETA: I have ridden collection before that, so I wasn't learning along with my horse like you are. I showed up to 2nd level in High School and got to take some lessons on Grand Prix schoolmasters. It was a wonderful experience that I wish I could duplicate again :-D.
And she's not your typical Dressage horse. She's a 15.2h Appy/QH that is halter bred (Impressive grandaughter, HYPP N/N) and built slightly downhill. She does better when she's leaner, 4.5-5 on the henneke scale. She seems to more easily collect and hold collection when she's lighter on her feet, since she's having to fight her conformation to get there.
My daughter wants to show her barrels and WP, so I need to get her back in shape before she starts her training for those. I plan on bringing her along with Dressage fundamentals. All of the Barrel horses I have known that were started that way ended up doing a LOT better than expected
. My mare was already trained for WP, before I decided all that glitz and $15,000 saddles just wasn't for me, lol. Plus she just hates loping... She'd much rather canter, or gallop, lol. Thankfully the youth WP classes my daughter wants to show in aren't very competitive that way. My girl has a GREAT slow, almost standing still jog (got even better once she learned Dressage!), but her lope is more like a slow or collected canter.
And no, western "collection" is NOTHING like Dressage Collection (with a capital "C"). Western collection is just slow, with shorter strides. It's not "true" collection in how we think of it in Dressage, where the horse's strides are not much shorter, but "slower" and more elevated. A good Dressage horse should have those back legs reaching well under him, but the front legs will be reaching a bit less, and all legs will be going slower. A good WP horse will have legs reaching 1/2 the regular distance of the "working" gait AND going much slower, with less
elevation. Dressage collection takes a lot of lower body movement to sit correctly, were WP collection takes almost no lower body movement to sit.
Anyway, I digress
. I look forward to reading other people's responses as well.