Everyone has a past. Unless you get into a relationship in which both parties are entering into their first relationship, it is likely that both people are bringing a certain amount of baggage to the table. For some people, baggage is hard to deal with. If your girlfriend's past bothers you, become proactive about dealing with it if you value the relationship. It is a bad mistake to shrug off the problem and assume that it will go away. If it worries you or makes you jealous, it will likely only become worse over time.
Remind yourself about all of the things that you've done over the years. Likely, she wouldn't be thrilled with every single one of them, either. Keep in mind that you both have baggage, and that judgment doesn't do either of you any good.
Remind yourself that if you can't deal with someone's past, that is your problem and not her's. If she is committed to you and working hard in the relationship, then she is already fulfilling her part of the relationship. Moving beyond her past is up to you, not her.
Avoid bringing up the issues once you have discussed them in depth. Once you've discussed the past, especially if it bothers you, try to just let it go. If you keep picking at it like a scab, eventually it will become a scar.
Look at yourself and wonder what it is in the relationship that you doubt or fear. Worries about the past are sometimes caused by issues present in the current day of the relationship. Examine the reality of the relationship and evaluate your real feelings instead of dwelling on the past.
Seek counselling if you still cannot forget your girlfriend's past. Some people have a more difficult time than others when letting go of a partner's history. If you value the relationship enough, it is worth seeking professional help to try and salvage it.