Just spent a weekend away with my girlfriend. We're both 25. We've had typically had very loving intimate sex in the past where it's been fine but for whatever reason this weekend I kept getting soft the moment I went to actually fuck - I'd be hard for ages ahead of time while we messed around, but the when the time came to actually enter her I'd go really soft. I'm fit and have never struggled with this before with previous girlfriends or her (with the exception of once the first time we had sex). Thinking it might be psychological - this happened a bunch of times on the first night to the point she got really upset and asked if I had lost attraction to her. On the second night after it began to happen she said "fuck me like you hate me" and I was immediately rock hard and we had amazing sex that night, the two of us came and we laughed about me being fucked in the head and spent the rest of the night cuddling. She's now super upset and sad, especially that it was the hatefucking/negativity aspect that got me going - she called it porn brained shit. She's right, but I haven't watched porn in 3 years, so I'm not sure what's going on with me. I love her so much and don't want to lose her, nor do I want to only be able to have sex with her like this - it's hot but I also want to have the loving intimate sex we've always had too.

She's said that as hot as it is in the moment that it makes her feel gross and sad that what got me going was that approach/thinking like that/verbalising any negative emotion or thought I have about her (even though during the actual event I just made up fake annoyances and it worked, it was like acting, didn't seem to make a difference and got me hard anyway). We theorised afterwards that it could be an ego thing that allowed that particular thing to work? Like a "fuck you, shut up, my shit works/there's nothing wrong with me" but again I'm not sure why this happened in the first place.

Any idea what the fuck is going on here and how I can fix this? I just want to be normal and have normal loving sex with my beautiful girlfriend :/. It's humiliating that it even lead to this and that we went through 3 nights of sex where I'd be hard right until I went to fuck her and then lose it unless it was through a lens of hatefucking, and then it'd be great, but now we both feel sad and weird about it. Emotionally we're extremely close so this has really thrown us off.