One of my best teachers used to say that two relationships is fewer than one. He was right. By constantly thinking about other possibilities, you prevent yourself from making a true commitment to your girlfriend. It may not be a big problem for you, but it will certainly be a huge problem for her if she commits to you and you can’t respond equally and fully.
My guess is that the betrayal by your former girlfriend had a bigger effect than you think. You may be protecting yourself from ever being hurt like that again by keeping a part of yourself out of your current relationship. Yes, you have forgiven. But you may have unconsciously (and wrongly) concluded from the experience that to truly love is too risky for you. For that reason, I do think it would be useful for you to see a therapist for a few sessions to help you grieve that old relationship and open yourself up again to loving someone completely.
I wish you well.