The best way is to work with your girlfriend on this issue. You may need the reassurance that your lack of confidence is not warranted. When she helps you see that this has no basis you may be able to move past the trust issue. At a point however you have to accept that lack of trust can ruin a relationship. Ask her to work with you to resolve the negative feelings you are having. Sometimes communication and open honesty can work best. If you learn to talk about issues as they come up then you don't obsess about issues that are all your stuff.
Please press accept