You say you have tried to forgive your girlfriend and perhaps you have. A key issue now is trust. You can forgive someone and still have trouble trusting in certain, specific areas. In your case, it may be fidelity in the relationship. Forgiveness is an unconditional response of goodness toward those who have hurt us. I say it is “unconditional” because every moral virtue we can name (justice, courage, kindness, and so forth) never requires that someone else does something before we choose to be just or courageous or kind. And, if forgiveness were conditional on an apology, you would be trapped in unforgiveness until the other said those two little words: “I apologize.”
Trust, in contrast, is part of reconciliation, which is always conditional—conditional on the other person???s remorse (an inner response), repentance (“I apologize”), and recompense of some kind. In this case, the recompense seems to be trustworthiness, which is earned one small step at a time. You seem to lack trust because of your statement about your feelings when your girlfriend interacts with others.
I recommend first going through the forgiveness process at least once more to be sure that you have forgiven. If you are still feeling resentful or uneasy, then from your position of having already forgiven her, consider whether or not you should discuss the theme of trust with your girlfriend. This needs to be done in love and in truth so that she does not feel condemned. The point is to talk about ways to build trust so that you can solidify the relationship.