I want to post this to try and help you improve. This is transferrable across not only games, but life as a whole. You have the gist right; there is no point in blaming others for a loss, and you can only control what you can do.
Here’s the problem, though. In a group effort scenario, it is pointless to blame yourself when there’s nothing you can do. There are failures in life that you just have to accept. There are many things you can learn from these experiences, and in WoW it is much more than just “I didn’t do enough damage.”
It’s fine to learn that you should communicate better, that you should be more careful who you take or who you join. And it’s also fine to just realize that a key was bricked and there just wasn’t anything you could do about it.
You aren’t a god among men. You aren’t able to right every wrong, and you’re not god’s gift to M+ either.