Has your doctor tried any birth control pills? They sometimes help with the reduction in cysts. A regular ultrasound sometimes will not pick up a cyst...a tranvaginal ultrasound is normally the best.
How long has it been since your last ultrasound? How long ago did your cyst rupture? Could the pain you are feeling be related to the ruptured cyst? Sometimes you can have lingering pain for up to 2 weeks with a ruptured cyst. If you are afraid that the doctors will not take you seriously, take you mom or another adult with you and have them tell the doctor how bad your pain has been. Let them know that just because you are 19 doesn't mean that you don't know your body. You know your body better than anyone. If it is telling you something is wrong, it probably is. Sometimes you have to be a pain in the rump before the doctors will listen to you. If that's what it takes to get some answers, then that is what you will have to do. Good luck, and let us know how you are doing.