I haven’t read through all of the comments, but I’m coming out the other side of being in exactly the same boat, so: it does get better.
My horse is coming seven (his actual birthday is in May). I’ve owned him since a week before he (truly) turned three, so it’ll be four years in April. I started riding him the first summer I owned him, largely just fifteen minutes of walk work a few times a week, and he hadn’t really started being asked to be a grownup until the last few months. He had some SI stuff going on that we injected in September this year after he bucked so hard during a lesson after the world’s tiniest cross-rail that I was told by my trainer there was not a chance in hell that I could’ve saved that one (the vet thinks it was an acute issue rather than a chronic one based on how he’s been at his last few chiro adjustments, so tbd if he’ll need to be injected again anytime soon, but we’ll see).
It’s been a real up and down with him, partly driven by the fact that we weren’t truly in a program until literally the last two weeks (I moved him to my trainer’s barn when she finally had a rare stall opening), partly by the fact that he’s large and gangly, and partly by my own psychological situation prompted by my bad experiences in a prior training situation (which predates him, I never would have been able to buy him had I still been there). The amount of improvement we’ve had in the span of a week (read: two lessons) is almost unbelievable.
He’s a saint, most of the time. He puts up with my ammy nonsense and the days when I’m not riding well. He’s incredibly smart (I swear he picks things up faster than I do most of the time), sometimes to the point where it’s to his detriment. He’s also got a real buck on him (even when it’s not SI-driven, it’s just how he likes to play, he did it to me the first time we picked up the canter last night after weather prevented turnout and thus his antics with his 3 and 4yo pasture-mates during the day) and once every six months or so he chooses to remind everyone that three of his four great-grandsires had reputations for being fire-breathing dragons. He’s got a fourteen-foot stride on him without trying and some days it’s all I can do to get him to stop.
Last night we were working on bend and transitions via a pole exercise and it was the first time that I’ve been able to package him up to that eleven-and-a-half foot stride that my trainer says he needs to be on without losing the canter. We went from him ripping through my half-halts in canter-trot transitions to sitting back on his butt and trotting quietly when asked instead of taking half the arena to get there. We suddenly have the ability to consistently get on the bit and I’m not looking like a complete fool when I ride. At least four times during my lesson when he was being absolutely wonderful because I was actually riding properly, my trainer just looked at me and went “He’s going to be okay.”
We’re taking a whole range of films in February when the performance vet is out next (for context: I bought him for $1500 sight unseen with basic flexions done and nothing else), just to get a baseline on everything and to give my trainer a better idea of what exercises we should be doing to help get his body in the best condition and give it as much support as possible where needed, but he just… feels better. He has his moments (like the testing phase we’ve had for the last three weeks where he’s suddenly forgotten how to stand at the mounting block ), but the patience we’ve had with him is paying off. There was more than one occasion at my last barn where I rode him around the indoor at night on my own after lessons were over and cried because I felt like we weren’t getting anywhere and I wasn’t good enough for him, but neither of those things were true. He just needed to grow up a bit, both physically and mentally, for all the work to really start to show.
I know it’s hard to hear give it time (especially when time might be two to three years), but… give it time, lol. I’ve had so many days where I’ve thought I’ve ruined my horse, but he’s also packed me around through so many situations that I wouldn’t trust other horses with even as a 4 and 5yo, and, as I said to my trainer last night when he immediately picked up on what we were doing, he knows what’s up, his rider is just incompetent sometimes so he doesn’t always get the support he needs. If you feel like your horse is a good fit aside from the baby brain, you’ll get there, and it’s okay if you need to cry about it or take a day or week here and there to just have fun with your horse instead of trying to accomplish things in the meantime.