I don't have any tips for you, but that seems to be the same for me (except for the meltdowns). It's like I either can do it or not, and if I can't, then no amount of trying is gonna make any difference. I just don't seem to improve.
I also have a problem with misconceptions: If I get the wrong idea (misspelling something or get a fact wrong) then it takes a long time for me to use the correct one. For instance the first time I spelled "icy" I spelled it "icey", and whenever I wrote it for a while (at least some months) I just couldn't remember the correct spelling. I knew I used to get it wrong, but I couldn't for the life of me remember which one was correct, and each time I picked icey because it looked right to me.