Definitely recommend getting someone else on him just to be able to see things from the ground - my main issue with my guy has been the canter, in that we absolutely could not hold a lead to save our lives. It was partly because of his SI situation, but when it still wasn’t fixed after the injections (though it was better), I eventually came to the realization that the issue was me and how incredibly crooked I had become in the tack (yay for four and a half years of bad coaching followed by four and a half years with six lessons total). I spent most of my December rides without a saddle to force myself to stop leaning and we still can’t reliably canter in a straight line but at least we aren’t swapping on circles anymore
I’d refer you to my Instagram to see the five hundred times that I said we almost had the canter together over the last two years, but it depresses me so I’m not going to do that hahaha. Been there, done that, got the t-shirt on making progress and then taking what feels like a hundred steps back. What makes me feel better is looking back on those early rides when we couldn’t even steer and realizing that we’ve made a lot of progress even if the incremental change from day to day isn’t always positive. We “regressed” a little bit tonight in that we were nowhere near as put together as our lesson yesterday, but we can canter a pole without charging it now and I count that as a win (it’s the little things).
Baby brain won’t go away overnight, but getting some training rides on him definitely could help if you feel like that’s what’s best for both of you. You being frustrated (whether it’s with him or yourself or both) won’t be productive for either of you (ask me how I know, lol) and getting that clear picture of what he’s doing and education for him from a pro may help you guys get over the hurdle. It’s not a failure to ask someone else for help if it’ll be a net positive for both you and your horse, it’s using all the tools in your toolbox and finding the best solution.
And really, the five/coming 6yo year is just A Time for them psychologically, before you even throw in any physical challenges. Half the time they’re grownups and half the time they might as well be two years old. My horse has settled down a ton in the last six(ish) months and I really notice it in our lessons - he used to get really worked up and frustrated when we would try new things that he didn’t understand, and now he gets a little tense but it seems to just be from him trying to work through the problem that’s been presented to him (as much as a horse can, anyway). A year ago we’d have to quit after forty minutes because his brain was fried and now we get through our full hour without any significant trouble. I can think of multiple horses (of various breeds) that I know personally that grew up a ton coming into their 7yo year - it just seems to be the time for a lot of them.