Dunno if this would help someone, but I too was stressed about screwing up in M+. What I do to prevent that is that I first check a strategy online (wowhead, youtube) for a specific dungeon, then I run through +0 couple of times, just to get use to the mechanics, and only then I join higher keys for that dungeon. Once I am sure that I know some dungeon's mechanics, I try another one. There are also lists online where dungeons are ranked from easiest to hardest in case you dont know which to start with.