"Doctors of course, are useless ... "
Not all doctors are worthless. Just the ones who don't really have an answer for your problem. I had a case of trigger finger once after doing a day's hard physical work in my older neighbor's yard. The orthopedic who first looked at the Xrays told me I had tight joints. I asked if I wouldn't have had those same tight joints before I did the yard work. He got frustrated and told me to come back after the hand specialist had come on staff. I did, and she diagnosed and treated the injury without a problem. So I would say you just have a doctor who sees lots of people with aches and pains and they don't have the time to spend finding out whether you actually have a specific condition or whether they can brush you off with a few words and move on to the next complainer.
Tendonitis is easily researched and I would encourage you to, at the least, be our own best advocate. Do the research to determine whether your symptoms actually match those of tendonitis. There are numerous issues which may be at the root of the problem you describe. I have a somewhat similar problem though my issue came about after a fall and it is more related to problems further up in my arm. Remember, your arm, wrist and hand are all connected by a series of levers and pulleys with many of those pieces existing inside a sheath that can also be inflamed. Make sure you are treating the right problem.
Most important in treating inflammation is resting the affected area. If you warm up slowly but then attack the rest of your practice as if there isn't a problem, you've defeated the rest period. Step back and take it easy until you can again play without pain. This may be a point where you need to consider your technique and, like a lot of players, begin to simplify and adjust how you approach your music.
Inflammation has many causes, some related to your diet, which is typically filled with 21st century additives which are unnatural to your body. You don't have to believe in "new age" anything to understand that every time you put something unnatural to your body inside your body, your body will fight back. Inflammation is the body's first reaction. Inflammation is a first sign that more serious conditions may follow unless you change tactics.
Treatment beyond rest is typically slow and sometimes a matter of finding what works for your body and your condition. Start with OTC anti-inflammatories which can be anything from NSAID's to natural sources such as Tumeric or Oregano Oil. All anti-inflammatories have some side effects so you simply need to find the one that's right for you. Despite your attitude toward doctors, it might be worth the time to find a holistic practitioner (DO's are better at this than are MD's) who can advise you on the do's and don'ts of anti-inflammatories.
More aggressive treatment begins with massage to the affected area. Look into a "TENS unit" and "transverse friction massage".
https://www.google.com/search?q=TENS+un ... e&ie=UTF-8
https://www.google.com/search?q=transve ... e&ie=UTF-8
Topical ointments can reduce some discomfort when you are not playing. It will probably be helpful to immobilize your finger while you sleep as many people actually work their hands hard during sleep. Your pharmacy probably has a soft "finger sleeve" with a gel insert which can be frozen for icing down after a work out or at night. There's also a device which you can wear during the day which is nearly invisible but provides support to your finger while largely immobilizing your finger. Just put "soft finger splints" into a search engine.
Nothing "out there" about either treatment, both have long been used in physical therapy. If you live in an area where there is an orthopedic doctor who specializes in treating the injuries musicians tend to suffer, give them a try.
Treating such a condition however requires a more holistic approach than just telling the patient to take some aspirin. Begin by looking at any changes to your posture and how you hold your guitar. Often, as pain creeps up on us, we begin to make changes to adjust the position of the guitar which actually leads to further damage being done longterm.
After that, make sure you are not creating more problems by bulling through the pain. Doing so only creates more inflammation which gets you no where. Rest is your best friend now. Take it easy when you play and consider taking longer periods away from the instrument. Icing down afterwards reduces the temporary flare up of inflammation but the damage has been done and is deeper than a temporary application of ice can address.
Mostly, if your doctor has no answer, do your research and find what works for you.
