You need to have several heart to heart conversations with your girlfriend. Start off by telling her what you told me - that you love her and that you want your relationship with her to continue. Explain to her that because you love her you want an exclusive relationship with her alone, and you need to be able to trust she wants the same and to know if she is willing to commit to you alone, to the exclusion of all other men.
Then go on to share with your girlfriend how you felt when you discovered she was seeing this other man - were you angry, sad, broken hearted, betrayed, lost, jealous, or a mixture of all these emotions? Share your feelings. Then explain to her how your feeling this way caused your heart to close to her and how you considered ending the relationship even though you loved her.
Ask her to respond to what you have said. Then listen carefully to what she says, without blaming her or judging her or making her wrong. Keep in mind your objective - to get back with your girlfriend and to be able to trust her again. During the conversation that develops be sure to ask her if there was something she felt was missing in your relationship that encouraged her to stray. If so I suggest you talk about what is missing and how by bringing what was missing into your relationship you can rekindle love and loyalty.
So listen with an open mind and an open heart without interrupting her or making her wrong. If you listen in this way, what your girlfriend says may well open your heart again and gradually you may find yourself able to trust her and to re-enter your relationship. If she speaks wisely and you listen intently, you may learn something valuable and be able to find it in your heart to forgive her.
Make it clear at the end of these conversations that you love her, that you want to be with her alone, to the exclusion of all other women (if that is indeed how you feel), and that you must have the same commitment from her if the relationship is to progress. Ask her to communicate with you if she is ever unhappy with the way the relationship is developing and explain to her that going off with other men will only lead to the destruction of trust between you and threaten all that you have built up together.
If you get a positive outcome from such conversations you can start anew and your relationship may even feel strengthened by what happened.
If you feel you need some help to heal these wounds I recommend Shared Couples Trance. You can relax together and listen to this CD - doing so after having the kind of conversation(s) I recommend will help heal those open wounds!
I appreciate you writing in to me about your personal difficulties and wish you all the best in resolving them.