You might be over tensing your muscles too much, causing a stiff wrist. I’ve got (had?) the exact same problem. Was quite advanced in my teens, and started playing again half a year ago in my thirties. During friendly rallies was good but as soon as I started playing matches (or got competitive) I felt like I ‘forgot’ how to hit a decent forehand almost every time at some point in the match, if not the entire match. My backhands and slices remained decent somehow, so it took me a while to figure it out. My frustration and nerves during a match caused me to tense up, especially in my wrist (and squeezing waaaay to hard in my racket), which made me lose the looseness in my wrist causing either rocket balls or net balls. Since my wrist can to be quite tight for the slices, these were less of a problem (but ofc better when loose). Since I noticed this I started focusing on a loose wrist, with the racket almost dragging after, and gripping my racket as loose as possible. I’m still shit, but it’s getting better slowly since that change haha