As the title says, some mistakes just stick no matter how long or how hard you study, or even regardless how good your German is overall.
As for me: I've been learning German for about five years, and I've been living in Germany for seven months now, but choosing the correct grammatical gender still feels pretty random sometimes. My native language also has grammatical genders, so the concept itself was never new to me, but it's still just too much to handle.
Even though I'm currently around C1.2, I keep struggling with „going to Rewe/Aldi/Lidl/other shop names“. Theoretically, I do know that it's „ich gehe zu Rewe/Aldi/Lidl“, but I keep mistakenly saying „nach“ almost every time. Actually, at this point I'm kinda stuck in a loop: I've been corrected so many times that I know that I'm likely to use a wrong form, but I keep forgetting which one is actually wrong.
Do you experience something similar? Grammar, word choice, pronunciation, anything?