It is on the big side, but if you were told it is functional cyst, it might go away on its own. If you have no pain, do another ultrasound in about 6 weeks and see if it gets smaller. You are less likely to get cysts if you are on birth control pills, but those carry other kinds of risks.
Marie is right on target. You need a cyst that size to come out. It can cause more pain and push on other organs if it gets too big. In the future you may ask your doctor if birth control pills would help keep your from getting cysts again. Sometimes they work and sometimes they don't. It is pretty much a try and see what happens thing. Did your doctor tell you what kind of cyst you have? Please know that we are here to support you and give you advice anytime we can. Please keep us updated on how you are doing.
Probably not. At that size, surgery is recommended. Prevention is not an option now. Now you are seeking a cure.
As for prevention? I suppose if we all lived back in the pre-industrial era before the invention of all the "what the heck is that?" type chemicals that are additives in our food, medicines, cosmetics, lotions, shampoos, food packaged and cooked in plastic, etc., etc. There are artifical hormones all over the place that our bodies are absorbing and our ovaries are responding abnormally to this junk.