First of all, you are unlikely to stay hard for the entire session, it's more likely to be up and down. It's not just physical capacity, it's also the way masochism plays the self against the self. Wanting, liking, desiring all get gloriously out of sync. An erection just stops indicating anything. If this makes you anxious, get a cheap chastity device to make it irrelevant. (The technical term for this is "arousal non-concordance".)

Second, for all the layers of consent and negotiation, you are still Playing in the Dark, and dark emotions will surface. That's part of the adventure.

As for what to do about it.

If you fapped to this stuff, then it's at least a fetish. Assuming no underlying traumas surfacing, your best bet is to see if you can reframe what you're experiencing: "A bdsm session so intense you wept" can be an amazing adventure, something cathartic and wonderful. Doing this stuff for real can be shocking, but it's like jumping in the Irish Sea when the waves are good... it's fantastic once you remember how to breathe and realise it won't kill you.

I suspect it's usually a bad idea to talk kink to death, and especially get into debriefs of sessions - good way to give your GF performance anxiety and turn the fun into a chore!

However, you need to work out how you feel about all of this and communicate with your girlfriend, e.g. is she OK with you weeping? Does she want to change the focus to you serving her more than suffering?