I had a nasty case of lateral epicondylitis (tennis elbow) for years. I tried stretching exercises, physiotherapy, massage, etc. It got so bad, I could barely hold a cup of coffee without dropping it and I was seriously considering surgery.
I decided to give exercises one more shot but rather than use the exercises from the physiotherapist, I used exercises that caused the most pain - reverse wrist curls and another exercise holding (pronated) the end of a dumbbell with a plate on one side and lifting to 90 degrees. I did 3 sets of each exercise eod starting with a light weight and increasing over time as strength increased. Sets were done for the maximum "burn" possible - usually 30 - 50 reps and the I concentrated on slow eccentrics (30 seconds) - anything to make it harder and increase the burn. I used NO stretching, ice or massage.
What I first noticed with this protocol was greatly reduced pain the following day which would slowly return until the next exercise day. After about 2 months, the improvement was quite significant and by 6 months it was probably 95% improved. I saw no further improvement beyond the 95% so I stopped the exercises.
The improvement has remained to this day although last winter, I got tendonitis in the other elbow. Maybe it just moved.
I'm not suggesting my experience is the answer but there is evidence eccentric loading is beneficial for tendonitis.
CBS