Hi, I'd like to help you with your question.
Jealousy is a painful and distressing emotion. It feels like it takes over your logical rational self and makes you do things you feel you cannot control. Many people struggle with jealousy and find it difficult to cope with.
Feeling jealous over your girlfriend's past is basically caused by your feeling that you and your relationship are threatened by that past. No matter if your girlfriend had one relationship or 100, you feel threatened by someone else being close to her.
When faced with this perceived threat, people who experience jealousy feel fear. They feel that since their partner could be with someone else in the past, they are not number one in their lives. They want to feel they are the only one so they do not feel insecure or risk humiliation if their partner leaves.
At some point in a relationship, almost everyone fears being abandoned. But when you feel jealous to the point you cannot let go, this means your fear is overwhelming and you do not feel worthy. Which says that your self esteem is low. Underneath, you feel you are unworthy of the relationship. This is usually caused by past issues resurfacing, such as an abusive childhood or other attachment/abandonment issues as a child. Or as a child you learned not to trust for some reason.
To help yourself deal with this issue, you need to address your self esteem and your ability to trust. Trusting someone takes faith. There are no guarantees. But in order to do this, you need to feel ok about yourself first. Therapy can help you overcome these feelings. Self help can too. Here are some resources to help you:
With some work, you can overcome these feelings and develop a deeper more satisfying relationship.
I hope this has helped,