Does anyone here/has anyone here ever suffered from Retroactive Jealousy...

good advice

its normal bro. nearly every guy has this worry and insecurity
its fucking engrained in our dna to be territorial like that

You posted an OCD psychology picture, why are you asking about body count.

My gf and I have both only been with one other person but she off handedly mentioned a sexual experience she had with him and I still get caught in this cycle thinking about it and it fucks me up. She tells me she's never felt the way I make her but I dunno if I can manage to keep that thought and the image of her with him in my head and believe. She's really honest but how do I know she's not just lying to me, she told me she faked orgasm for him and didn't even know what they felt like but all of that could be bullshit to sooth my ego since she wants to get very serious with me.

I have no solutions

Why is she talking about previous sexual experiences with her current boyfriend? Moreover, why are you tolerating it?

this is the best advice. i've been through something similar with my bf, knowing he was in the process of kinda pursuing a girl before we met. still can make me feel a bit insecure if i let it. but if you let that insecurity get in the way of your happiness, it can really poison a relationship. try to value the present and future of you two together more than the past, before they knew you existed and what you had to offer

I was asking her a dumb specific sexual question like "Have you ever fucked on the floor" and she answered that yes but it started in the shower. That seems like good sex to me, and that doesn't gel with what she was telling me and now I'm looking for any other inconsistencies. Admittedly she knew she fucked up and was really upset about it and I've told her about sexual experiences I've had before her without thinking.

you should probably just sit down and be open about it, try to really calmly tell her that personally you just don't really feel comfortable hearing details about a past relationship bc it hurts

Ok. If she had just been outright bragging about her past to you unprompted then it would be different. There was a thread about that a few weeks ago on here and everyone told the guy to dump the girl.

Asking these questions never ends well...personal experience here too

you're mad at her and looking for reasons to distrust her and completely sabotage what could be a good relationship, when you're the one who asked the question