I am uncomfortable with the dentists telling you to leave something alone that is causing pain. This does not seem logical to me. I would get a new opinion from a new dentist. This time pick either a periodontist, or an oral and maxillofacial surgeon. Be sure that they take new radiographs of the area to compare to the old x-rays to see what has changed. If they believed that it is a cyst, I am unsure why they did blood work, since nothing out of the ordinary would show up from a benign cyst. If they thought that it was an abcess, then blood work would show elevated white blood cells and other immune system responses to an infection. But you don't leave an abcess alone. So from your description this seems rather unusual. In any case, since it has not resolved in a year, and is causing pain, this is something that a more qualified person should take a look at.