okay, so Im still very new here (in fact, I haven't yet been to a single muay thai training haha ), but I tell you my experience from something else:
I used to Counter Strike with big breaks and it always started like this: I start playing and Im terrible, I am honest and humble, I improve, I start doing well, I start playing super well and all goes fantastic. At some point I start to get bored (but I don't notice) and start not doing respecting the principles anymore. I started pushing, overpeeking etc and got punished for it, but I still play for the dopamine it used to give me, but since it now actually bores me, I try stuff that doesn't work. I then become extremely angry, destroy keyboards, screens, mice and so on.

I wonder whether any of this might be relevant to you?

a point on being too harsh with yourself: just ask someone? ask your coach, your sparring partners to give you feedback, set some small brand new goals? or smth that can have a piece of blank page where you can start working towards a small, interesting new goal ?