Sounds like you need to manage your loads, mate. Patellar tendinitis and patellofemoral pain are both overuse issues.
No amount of magic supplements are going to help you with that. And no amount of "sliding back foot lunges" are going to help you if you then go play football games.
In short (assuming you want to fix it):
1. Stop all exercise/activity that aggravates your symptoms. No football games, no sitting/sleeping with your knee flexed, etc.
2. Supplement with unloaded mobilisation (flexing/extending inside a pool would be best, doing it while lying down will work, pedalling on a stationary bike with minimal/no resistance might also work).
3. Do any movement or activity that involves your knees but produces no symptoms (if walking produces no symptoms, then try to do it often/do more of that, even if you're not sure it helps; if leg curls don't hurt, then do them often; etc.).
4. Allow for a few days of deload/inactivity for your pain to reach to baseline, then start re-introducing loads in a very gradual and very controlled manner, and be patient with it until you manage to build tissue tolerance. "Loads" could be anything from squatting (progress from high box squats to full squats, from hip dominant to quad dominant variations, from light to heavy weights, and from small to large session/week total volumes), to other leg exercises, to explosive lifts/jumps, to running/sprinting. Progress from very low loads (volume, intensity and biomechanical variations) to high loads. When building tissue tolerance, the rule of thumb is that any load that produces pain/aggravation is ok as long as the pain has returned to baseline within 24-48 hours (if the pain remains elevated for longer then the stimuli was more than your tissues could handle and it probably did more hard than good). Wait until the pain has returned to baseline before the next stimuli.
Keep in mind that articular cartilage synthesis is a very slow process. Depending on the severity, it can take weeks, months or years (and, of course, if the damage is extremely advanced, it might never happen).