Hopefully no one walked in on that
Good that its working though.
Here's my 2 cents - not a PT, doctor or anything so take it as just an opinion. Is it definitely an impingment problem? I guess you havent always had this if it started in 18 but I think impingment problems can always be solved. If you think about it, the PRP is only going to heal any existing fraying or inflammation there but if you're not fixing the cause of the problem its going to happen again.
Impingement of the tendons in that area are due to a reduced space in the AC joint: simple mechanics. We know that people with naturally reduced space in that joint are more prone to impingement but space also decreases in there because of muscular imbalances. In other words, something is tighter thats squeezing the space where the supraspinatus tendon runs and it is feeling the pinch. Its not the tendon's fault, its your surrounding muscles. Unless there's been a direct injury to the area, thats highly highly likely to be the reason. I reckon your whole shoulder region is tight, causing your humeral head to sit too high and forward, squeezing the AC joint. Its a small space so only a fraction can make a difference.
So what to do- this is where Im a little weak so these are only suggestions You need to work on getting that humerus down and neutral (further back). You do this by getting better scapula movements mechanics, building up rotator cuff muscles, dead hangs of pull-up bars, heavy as fuck farmer walks/holds and general mobility work. All of these, not individual elements. Also, trigger points in traps, scapula region and lower attachments of the rotator cuff muscles - smash them with a hard ball.