My CA-125 was slightly elevated`--`just outside of the normal range. My cyst was 6cm and my OBGYN was pretty confident it was just an endometrioma...and it was. The surgery was my first and it was pretty easy. At your age, I wouldn't be worried about ovarian cancer. Lots of things elevate the CA-125. Sounds like you are in good hands. Everything will be fine, you'll see.
Oncologists are not surgeons. Relax - you are seeing the correct type of doctor for this surgery. This is probably a benign cyst. Worrying about it won't help, so just decide it's a benign cyst until you hear otherwise.