The more you play, the more you practice, the more you study, the better you'll become. Don't expect instant results, though. Dedication, time and patience is all you need. Admitting that there's always room for improvement is something that even the best of the bests think. It's a long road for the vast majority.

Is it possible that I'm being too harsh on myself?
Yes, as always. If you're not going to use it as a tool for improvement, then it's gonna cause more harm than good.

The other question is, is it possible to put too much pressure on yourself, whether it's in casual or raiding content?
Yes. When you ask me to check your DPS in normal, then you're taking it too seriously and you're with that constant pressure of "I need to do this perfectly". Line-up matters, kill speed matters, buff alignment matters, knowing the fight like the back of your hand matters, having the right gear matters. These are the things you need to take into consideration before you pull the trigger and snap at yourself.

I honestly don't know what you're expecting. Like I told you before, you won't become amazing overnight. Like I said above, it's a long road and it will take quite a bit of time and practice.

As a personal experience, as you may know, I keep all that drama for myself. When I have a terrible raid night I may dread it a little but withdraw, study the streamings, review the guides, talk to my static and I aim to address the issues that were there. If there's something I'm missing, I ask for their feedback. Group effort is group effort, and they are the first ones that should know about your concerns. If they are okay with everything, as long as you all clear, then it's all gucci, call it a day and go sleep in peace.