What Causes Painful Lump On The Roof Of Mouth?

I have a lump/cyst on the roof of my mouth. It has been off and on for years. Have been to a dentist, however because it's reaccuring , they did 3 root canals. None of which took the painful lump away. It can get to the size of a quarter and hangs down. It's extremely painful and sometimes will bust open. It's normally filled with blood but has had green, yellow puss with a horrible odor. It has even busted and filled my whole hand up. The infection can be seen up to my eyes in X-rays. I'm thinking I may have to have it surgically removed because the pain can be very intense. The dentist can not believe I have put up with this pain for so long. Any kind of advice would be helpful. I would like to maybe have a understanding of whAt it may be. Thank you