I am wondering if anyone has any suggestions for dealing with tendonitis as a pianist. I have discussed this issue with my piano teacher, and we have concluded that the injury is most likely due to excessive tension when I play. This is very frustrating, since I’ve already taken about 2 months off from rigorous practice due to this. Any advice would be much appreciated!
I have multiple professional string player friends who have found acupuncture treatments very effective.
Long term, you need healthier technique and practice routines. But it sounds like you and your teacher have figured that out already. This will probably mean re-working a lot of your technique and playing habits, possibly starting with music well below your current level until new, better habits are established. Also, incorporating rest periods into the practice sessions and planning better for a productive practice with real problem solving- not just going on and on with the repetitions.
There are physicians who specialize in repetitive motion injuries and some who work with musicians. Perhaps you can find one in your area. Sometimes, they will suggest stretches that can help, analyze your playing approach and suggest changes in posture or body usage for healthier muscle usage, etc. You might also look for someone trained in Alexander Technique- usage of the body focusing on performers and athletes. Very helpful for becoming self aware of tension and muscle usage.
Good luck, very discouraging situation I know!
Momofadult’s answer was very thorough. Just wanted to add that there are also physical (or occupational) therapists who specialize in helping musicians. An MD can probably direct you to one, or perhaps Dr. Google.
My son got tendontitis from computer use (he works in CS) and working on ergonomics did help him.
Finally, look into PRP (platelet rich plasma) if the problem becomes severe. It is a form of prolotherapy involving drawing blood, concentrating the platelets by spinning, and injecting to regenerate soft tissues. https://www.verywell.com/platelet-rich-plasma-prp-injections-2549576
Corticosteroid injections can help temporarily but PRP can truly heal. You still have to pay out of pocket for it but it is offered in mainstream, prestigious facilities.
Should I continue to practice through the pain (with tact, of course)? Or rest until all pain is completely gone?
There are clinics in hospitals in some urban areas which deal specifically with musicians’ injuries: Cleveland, Boston, Baltimore, NY, etc. Look online and see if there is one near you.
Standard advice is to rest completely. No practice with pain. Back in the dark ages of my studies, multiple friends (string players) were forced to take weeks or even a full semester off to recover. One of my kids had a conservatory friend out for an entire year. You would think we would learn, obviously not.
The PRP procedure that I described regenerates and heals inflamed soft tissue very quickly. Potentially days. I would look into it. You would still have to work on ergonomics and technique to avoid recurrence.
And try to find an MD and/or PT who treats musicians asap 
This is tough. But it may not have to go on too long if you can find the right help. As you know, however difficult this is in the short term, you want to preserve the ability to play long term.
Make sure you’re really moving your arms and doing lots of gestures with your wrists especially, they help relieve the tension so it can’t spread to your hands. I had a tension problem for a very long time, when you feel yourself getting tense just stop and shake your hands out. Remember not to press into the keys too, rather let the momentum/gravity of the movement of your wrists and arms allow you to play the keys. Just some tips my teacher has taught me, hope it helps.
I have been resting as recommended, but have not been able to find musicians’ clinics nearby. So I have a performance this weekend (piano duet) that is a bit technical… I am wondering if I should tough it out? I really don’t want to let down my duet partner.
Honestly, I would be really concerned about your current teacher’s priorities and techniques if you got this way under his or her tutelage. I’d consider looking at new teachers if you are hoping to go into music. You can find doctors and PT with direct experience with this type of injury especially near larger cities. I know we have some in the Minneapolis area. So I’d be making calls about that too.
^^^ That.
Well, I am partly to blame in the fact that I did not bring this issue up to my other piano teacher (who is organizing this performance). Correction - probably completely to blame.
Will I do too much damage by continuing to practice up until Saturday?? This is a really difficult situation.
It seems there is a short term issue and a long term issue here. The short term issue might require rest, PT or maybe PRP. The long term issue requires changing technique, and maybe teachers.
I’ll only mention this once more, but I told a friend who was having wrist issues about Platelet Rich Plasma and she just had PRP with great results. A family member had it in knee and SI joint with miraculous results.
Find a physiatrist who knows about musician’s challenges and discuss treatment in the short term and prevention in the long term.
If you were my child, I would tell you to quit the practice. You can make things worse.
Lang Lang is on an extended break currently - injury. I remember the comment from one of my professional pianist friends when she first saw him perform as his career was just taking off. She predicted then that he wouldn’t last long - way too much tension and strange fingering. Sad.
You don’t say what level you’re at, but don’t take this lightly. I have been told by more than one person that there are currently more career ending injuries to college musicians than college athletes. Not to say this will threaten your career, but you should definitely take the long term view.
How was your duet concert? You should check “Taubman Approach” out! The approach is known for helping especially injured pianists like you.