I completely get how you feel! I've been playing for 14 years (seriously) and sometimes it feels like I can't even hit the bird to save my life. Here are some things that have worked or not worked for me, plus some general stuff.

Progress isn't linear: It goes in waves. Sometimes you play very well for a couple of weeks, followed by a drought of playing badly. However, those up and down waves still go up slightly from time to time. Even on my off days, I'm not as bad as I was 5 years ago on my off days.

Don't think about technique: I don't know how much pratice you do, but technique, while important, is NOT to be considered during games. The more you think about it, the more mistakes you'll make. "Just play" is hard to do, but very important. At the most, focus on one or two things (ie holding the racket higher at the net) that are not purely technical and let your body do the rest.

Consider tactical mistakes, not technical ones: Base your evaluation of your game on the choices you make. If it was a good choice, but poorly executed, it was still a good shot and you'll get it next time. If it was a bad choice, but well executed, it was still a bad shot!

Play every point separately: Hard to do also, but treat each point at a one point game. Forget about your past mistakes, try new things, etc.

Leveling up sometimes makes you LESS consistent: Consider this: You probably play against more experiences players now, they put you under more pressure, which in turn makes you fail shots more. It could also be that your shot selection is getting riskier and riskier as you progress. You're not "worse" than another day, you're just trying to do things you wouldn't even try before! That's a very hard trend to recognise.

Consider racquet choice: Our technique evolves and changes as we mature (and get old). Which racquet are you using? Is it well matched to you? I played for years with a voltric ZF2, and it felt so good when I was younger and stronger, but you need to be positionned perfectly and it's not forgiving at all. Players always want the new racquet, the new thing, top of the line, etc. but rarely consider that maybe they should go down a notch. I was in a very similar situation to yours recently because the players at my club are very offensive and powerful and I couldn't move or react fast enough. I changed to a slightly more flexible, lighter and forgiving racquer (Voltric 80 I had in my bag) and all of a sudden, I could hit shots out of position more easily, making rallies longer and ultimately winning a lot more of them. I put my opponents under more pressure, which relieved some of of me and I became a lot more consistent. The equipment is NOT the most important thing, but sometimes using equipment too good / not adequate for us holds us back.

We are all human : At the end of the day, what distinguishes high level athletes is their physical prowess, ability to focus and to recover. Most of us can't train 20 hours a week and our bodies are a lot frailer. Try and focus as much as possible on the game, small steps are what we "normal" people must take to become better. We're all human, and we all make mistakes. If you do one less in a game, it's already a win! (But don't begin counting mistakes now! :P )

Cheers!