It's natural to feel jealousy about someone that you love but you can't let these emotions get out of control and become obsessively destructive to your own well being. That is not healthy. Your anxieties, fears and jealousy are most likely due to your own insecurities. If you really love your girlfriend it shouldn't matter about what has happened in her past because she is with you now. The past is history so focus on now and don't allow these feelings to interfere with your current relationship. If you are finding them difficult to cope with I would suggest some therapy to help you deal with these feelings. All the best.