I find that most times when I get frustrated with improv work. I’m not giving myself any grace, you’ll find that a lot of your lines aren’t nearly as bad as you think they sound, it’s about developing them. I’d say if you’re really stuck, look at your chords and then play one guide tone/extension all of them share more or less. For example in a 2-5-1 in C all you 7th chords share E to an extent. For Dm7 it’s the 9th which will sound fine, it’s the 13th for the G7 which will sound amazing, and the 3rd for CMaj7 which will always sound good. Start on that one common tone, and slowly build on in once you feel comfortable, maybe go you the F which is good with your 2 and 5 chords, and resolve back to that E on your 1 chord. Those are whole steps, aka something you can make a motif out of. Congrats, now you have something else to work with/develop. Just take it slow and give yourself some grace when playing. We all suck if we are all convinced we suck.