I've felt like this before, and i ended up switching facilities and trainers multiple times. Eventually i found the perfect place for me. The training is excellent, and the atmosphere & people are very encouraging. It's very helpful to be in a good environment where you feel confident, it's a good motivator. So, i would suggest switching where you ride or your trainer. It worked well for me
It's normal to feel sometimes that you don't progress. Horse riding is a difficult sport when you can progress a lot for a while and then suddenly stagnate. It happens to every riders!
You should settle new goals (do a new dressage movement, ride a horse you don't know, jump higher fences, ...) and work hard to reach them.
Talk to your trainer about it, I'm sure he'll understand!
Don't give up and remember that the most important is to enjoy rinding and having good time!
Hope this helps :)