Hello, I'd like to help you with your question.
It can be hurtful to look at your partner's past and think about her being with other men. You love your girlfriend and want to feel she is only with you. Picturing her with someone else can bring on feelings of jealousy and pain.
However, you are reacting to former relationships your girlfriend has had that were not only before she knew you, but are long over. If she is very supportive of you now and loves you, then there is nothing to be concerned about in your relationship. So that brings the question as to why you are experiencing these feelings if there is no current cause for them.
When you have feelings like anxiety
over past relationships that usually indicates that you are experiencing insecurity. Insecurity is usually the result of a low self esteem. Feeling that even past relationships can threaten you says that you don't feel you are secure in your relationship. It also indicates that you feel you are less worthy than your girlfriend's past boyfriends. Therefore, you are not secure with her.
Low self esteem and insecurity can stem from childhood. Bullying
, abuse or trauma are possible causes. Growing up not feeling you are worthy and loved can create doubt that anyone can love you and even if they don't, it doesn't affect who you are inside.
To deal with these feelings, you can address them in several ways. You may want to start learning more about your feelings of insecurity and how you can address them so you can see this situation more clearly. Therapy is a great way to address how you feel and get to the root of it. To find a therapist, talk to your doctor. Or search on line at http://therapists.psychologytoday.com/rms/prof_search.php.
You can also address your issues through self help. Here are some resources to help:
Relationship Saboteurs: Overcoming the Ten Behaviors that Undermine Love by Randi Gunther
The Courage to Trust: A Guide to Building Deep and Lasting Relationships by Cynthia Lynn Wall LCSW and Sue Patton Thoele
Also, work with your girlfriend to talk about your feelings and accept that this is about you and not her. She may end up feeling guilty because she doesn't want to hurt you with her past. But there is little either of you can do about it. So focus on the future. Let her know that you may need some reassurance from her once in a while and talk to her when you feel particularly concerned. She can reassure you that all is well between you. She can also help you work through these feelings by being a sounding board and offering her support.
I hope this has helped you,