One of the simplest pieces of advice I've ever seen, and one I don't see enough, is to be kind to yourself. Critique can be harsh. Critique can be rough. That doesn't mean that the whole process of critique should be so though. Quite often people try to defend things as criticism when they're not criticism, they're insults. You need to know the difference. You need to hold yourself to giving yourself critique, as opposed to insults.
Critique vs. Insult
For example, "I could have done a better job with the accent on Key NPC A" is good critique. Especially if you follow it up with specific points you think you could do better on (practicing the accent, imaginging some possible exchanges they could have, etc.) "How I handled NPC A sucked." Is not critique.
Yes, there is some critique in that second sentence - hence why people get confused - but there is no information in that sentence that is actionable. You don't know why it sucked. you don't know how it sucked. Worst of all, you've taken the whole thing - the whole portrayal of the character - and branded it all with the same mark.
Insult Free Zone
When reviewing what you want to improve, you need to keep insults out of the picture. Things like X sucked aren't helpful. All it is going to do is put you down and make you feel worse. Trust me, feeling like shit is not going to help you improve at anything. All it's going to do is make you worse - or even worse, it'll make you want to quit.
So if you need to phrase something as X Sucked, then make sure you can - and do - answer the Why. X sucked because I jumped the gun with it, I should have been more patient. X sucked because I completely flubbed my delivery of that threat, ruining the immersion. Those can work. At the least you have something to work with.
Accept You Don't Control Every Aspect
In a recent game I left feeling very poorly about my GMing because a couple key moments felt very phoned in. It didn't help that those scenes relied on skills I feel are still very weak, and I beat myself up pretty hard about it. The thing is though, when I reviewed the session in my head while part of the problem was my fault, part of it was out of my control.
What do you do when mid dramatic moment someone at the table makes an OOC comment unrelated to the game and suddenly everyone is out of the game world and back in the real world? It breaks the mood for everyone, and for the GM trying to deliver a key moment that's a death sentence for the scene. It sucks, but there's not much you can do about it.
Looking at that during self improvement there is stuff there to work on - like communicating to the players you'd appreciate no breaking from the game during tense moments and re-committing everyone to that idea - but that doesn't mean your flubbing the scene in question is because you failed to deliver. Something else happened. And that needs to be acknowledged.
Find The Good, Always
Finally, you need to make sure to find the good. I don't care how bad you botched a session you did something right. Even if it's just "I showed up and tried to run the game" it's something. Find the good. Find all the good. Remind yourself of the good things. Use them to beat back the sense of doing poorly. Improvement isn't about something being bad, it's about wanting something to be better - even if it is already good.
So go over those good things. Sandwich them around the bad. And keep it all in mind.
It should look something like:
- The Warlock's dream sequence went over well, and I had the right music queued up for it at the time.
- The Patron didn't come across as big and serious enough. It needs to have more power, more gravity to it. I need to be patient. It's a being of immense power and immortality, not someone worried about missing a sale.
- The reveal of the secondary villain happened with near perfect timing, and interrupting combat midround to do so made it work even better and the players seemed to like that.
See how it works?
Good. Employ it on yourself. It helps.