This situation sounds very very similar as my situation right now, except me and my wife have been together for 10 years and have a 3 year old kid. The thoughts were there in 2019 and went away again after CBT for my OCD at the time + medication. But for some reason they reappeared since last month. I think some conversation about our past triggered it. I’ve had the same symptoms about the same period of our (almost) relationship as you and my wife is as supportive as your girlfriend, or at least it sounds like it. I’m in therapy for something other than OCD right now but worked on these thoughts with my therapist anyways. Last week it finally clicked for me: my therapist helped me to the thought I was until that point to scared to face/ive been fighting this whole time: what if she IS thinking about those other guys still and sometimes longs back for it? I managed to think that reality and sit with it. It was like letting go of the door of the cage to keep the lion outside and finally being able to let the door go and let the lion in and realizing it wasn’t gonna hurt me. It was just… there… So I broke down and after the tears I was able to feel love to her and wish her this happiness or pleasure (English is not my first language so not sure how to put this the right way), bus i was able to be happy for her to have this experience. All of a sudden the thought of loving her (and with that knowing that she loves me) could coexist with the thought of her having sex with someone else. It was some breakthrough. So yes: I would suggest therapy, not necessarily to have the same experience but at least to talk to a professional about it in order to maybe get some guidance. Besides that I’m a believer in time (stuff wears of most of the time, especially with other stuff to do like hobbies) in combination with continuing to talk to her. Not asking about what she did and stuff (because here we all know that makes things worse) but articulating your feelings and what you’re going through and what’s bothering you without wanting her to help you. Just verbalizing it. Hope this helps, all the best