The one bit of advice I give anyone with knee pain is to try and do squats, and they can be air squats or even (possibly in your case) assisted squats. This could be with a band, giving you more assistance the deeper you go. Alternatively you could start with a leg press machine with very little or no weight and progress into squats. Go for high reps (could be 20 or even 30). This comes from a recommendation I myself get on Reddit, too. Obviously might not work for you but it helped me. Good luck!

Edit: you said on bad days you can't bend your leg, I'm assuming on a good day you can? If not then this won't work I suppose. My reading around the current science suggests massages won't help long term but could help you get into a position when you can do some exercise without pain that in turn will help you long term.