Hello, your tendonitis has probably become a chronic condition if rest has not helped. this is refered to tendonosis, because your tendon has slightly degenerated, which is why rest is not healing it. If you want to shell out a lot of money for an official diagnosis you can see the denigration and subsequent thickening of the tendon with ultrasound.
To be blunt, this is a shitty injury to have. I am a hockey goalie, and I’ve broken bones and torn muscles, had people step on me with their stupid skates etc. etc. and chronic tendonitis was the absolute bane of my existence when I got it in both quadriceps, it sidelined me from hockey for almost 8 months. What fixed it for me was strengthening my hamstrings and hip abductors. I also rolled out my IT band daily and was careful to stretch out everything in my lower body (my hip flexors and hamstrings were tight and this made it worse.) Other methods such as voodoo bands helped but did not solve the problem, which originated in muscle imbalances caused by playing goalie in a knock-kneed stance and not balancing this out with other work. Once my muscle imbalance issues were resolved I started an eccentric loading program using the stupid leg extension machine and a 1 leg press and some 1 leg squats and stability stuff when I fixed my knees enough to start those.
I would find a physical therapist if you can or get someone who can asses your strength/flexibility. I guess you might have a good idea of this yourself though. when you get to the last part with the eccentric loading make sure that you spend a lot of time on the eccentric part (duh) and use both legs for the concentric portion of the lift. this should hurt, but in a “good” way. For me it felt different from the initial injury, which burnt more. I know this is vague but you really should be able to feel a difference. be warned, I had to start the leg extensions/leg presses at a VERY light weight. I think it was like 20lbs no joke. It was humbling, but I had reached a point where i needed to use my arms to get out of chairs and you are probably not at that point I hope! Anyway my advice if you are doing this alone is to start with an embarrassingly light light weight and add it on super slowly until you notice the good pain turn into bad pain. do the exercises like 1x/2x a week, or whenever you feel ready. my knees always felt torn up after a lift so it would take me a few days to recover, sometimes i would only go 1x a week. remember, you are healing a tendon not a muscle which takes a bit longer so be patient! When I got to the point where I could play with only the good pain and not the bad I got back on the ice and never looked back. keep going the muscle balancing stuff though obviously.
e:also i forgot, don’t take NSAIDs for a long time! the block beneficial inflammatory responses and can inhibit healing if you take them for a really long time.