Guilt over masturbatory fantasy - girlfriends friend, ROCD

This happened about 1.5 years ago when me and my gf were living apart due to covid, there was a few times while masturbating that the thought of her best friend (who was also my friend, and my best friends girlfriend, we all lived together for a year during uni) would enter my head and sometimes I would entertain the fantasy a bit in my head, sometimes thinking about her and my girlfriend together or the three of us however, on 1 occasion in the heat of the moment I actually brought up a (non-explicit) photo of the three of us and noticed her cleavage in it.

I definitely felt gross and ashamed after and immediately recognised that i honestly wasn’t even that attracted to her, it was like I gave in to sexual impulse in the moment in the middle of masturbating. My memory on the incident isn't fully clear either and I have a feeling I might have actually stopped looking at it during it and switched to something else. I haven’t done this since obviously and sort of forgot about it but looking back on it now I feel like this was really inappropriate and disrespectful behaviour and it's sticking in my head like all other intrusive thoughts/OCD themes have in the past for me.

We’ve recently actually talked about boundaries in our relationship and fantasies etc and agreed that fantasising about other people is 100% OK but if it came to sexually fantasising about our best friends then that’s a bit close to home and would be really inappropriate and we agreed with each other that while it wouldn’t be cheating or anything unforgivable, it would need to be talked about and stopped. However in this moment I honestly didn’t remember that I’d done that before because it was 1.5 years ago.

How do I deal with this mentally and move forward? Me and my gf are very happy going over 4 years and while I knew she’d be disgusted by the act and maybe hurt by it I don’t think it’s worth upsetting her and potentially harming the relationship by bringing it up.

For what it’s worth neither of us are friends with this girl now but we were then. It's been on my mind so much over the past few weeks and the opinions of people online range from “fantasies are normal, stop worrying” to “that’s absolutely disgusting and you need to tell her and let her dump you” from the same post.

I appreciate any opinions and advice.

I fall into the “ fantasies are normal stop worrying about it“ crowd. What is concerning is putting so much emphasis on it this far after the event and worrying about it. It sounds like you and your girlfriend have a healthy sexual relationship, where you can talk about things. Try and focus on the future.

When we obsess in an unhealthy way, the emotional anxiety outweighs and overtakes the rational “facts” of the obsession and becomes destructive. Maybe that is what is happening here. I have OCD and that is the way it happens for me,