Ade, I have a similar injury to yours and like you assumed it was tendonitis. Finally after 4 months of resting, icing, and working the opposing muscle groups I went to the doctor, where to my surprise I found out it was not tendonitis, instead it ends up that my c7 nerve is pinched, a condition that occurs often and has symptons almost identical to elbow tendonitis(on the inside as you described). Treatment included some muscle massage, in the upper back area, from a physical thearpist and some funky stretches. This improved the situation by probably 80% after only a couple of visits. Might be worth checking it out. Good luck.
ps Hope I didn't post this twice, having problems posting.