How can I recover from tendonitis and get the painful symptoms under control?

Rest... Tendonitis, depending on the location, is usually treated best with rest and anti-inflammatory medications (motrin, aleve, (naproxen) etc). Some tendons can be injected with rapid relief, but that does have some risk as well. See your doctor and come up with a good plan.
Many Possibilities. Prolotherapy typically by itself will cause ligaments to tighten up, usually due to scarring or fibrosis of the ligaments. However now prolotherapy is used to introduce new blood supply to the area and then many times following it with the prp and/or stem cells in the area can repair the tissue/ligaments by the hopes that the stem cells regenerate the same normal tissue type. Regenexx.Com.
Diagnosis first. Make sure you have the proper diagnosis first as often there can be other issues such as arthritis stresssfracture as you previously asked or other issues that can confound such as nerve irritation, proper splinting if needed. Nsaids. Ice rest stretching and other modalities can help and see someone who specializes in that anatomy region.