Never been this confused from sex, I need some help figuring out what happened

There's a lot of stuff about what happened a few nights ago that is twisting my brain in knots, so anything you can do to help would be great but I don't even know what I need.

I'm an 18 year old healthy male in a long-term relationship with someone I love and trust (we're at 3 years and counting), but I fucked up a few nights ago by hanging out with a girl (let's call her Jude) who I know my girlfriend wouldn't want me hanging out with (Jude has a reputation for promiscuity). I spent most of the day with Jude, and met a lot of her friends who I really liked. She knows the university I'm at like the back of her hand, and it was great having someone let me into all the social circles.

About midway through the day, Jude gave me some jello shots. I handle alcohol well, so I was fine with that. She gave me some rum and I drank that as well. After talking for a bit she asked how I felt, and I said "a little buzzed." She said she wanted me more than buzzed, and I just played along and drank some more in between smoking some marijuana with her.

After a while she started kissing me, and I was so enamored by this attractive, popular girl that I just let her do it despite knowing my girlfriend would want me to stop her. She asked me to take my cloths off, and at that point I still had enough sense to tell her that I couldn't have sex with her. We talked for a good half hour about why I wasn't comfortable with it, and eventually she said she understood and she'd leave it be.

Fast forward a few hours, and she's still pouring me drinks in her room well past midnight. She kept asking me to have sex with her, and I guess eventually I gave in. I told her I'd do it if she were on the pill and we used a condom, and she agreed and confirmed she'd kept up with her birth control. I was terrified of getting her pregnant and felt guilty for cheating on my girlfriend, so I couldn't keep an erection. At several points in the 2 hour session I asked to stop because I couldn't get into it, but she wasn't satisfied and I didn't want her to think I was bad in bed so I kept trying. Not doing well by this girl would basically ruin my chances with other girls here; she tells everyone about how good (or bad) her partners are.

After a while, she had me take the condom off. I thought she was saying she was done, but then she pulled me up against her and we started up again. We tried everything we could think of to get me hard, and ended up doing some light bondage on her (one of my biggest turn-ons). We managed to finish me that way, but I realized I wasn't wearing a condom so pulled out and came on her stomach. At this point she admitted that she hadn't actually been taking her birth control, but I was exhausted and emotional so I didn't really comprehend it and we fell asleep.

I haven't told anyone about this yet. I feel betrayed, but I know she never physically forced me into anything. I knew what she wanted from me, and I never exercised my right to walk out and I even put effort into making sure she enjoyed it. I've still been hanging out with Jude, but only in public. I've been avoiding my own girlfriend out of guilt and knowing I won't be able to wrap my brain around my emotions surrounding sex right now, so I won't be able to give her the pleasure she's used to. I afraid I'm just trying to justify cheating on my girlfriend, hence the throwaway.

If I am just being a wimp, you guys can tell me. I'd rather that to this weird confusion, or to believing something that's not true.

I don't know how to put a tl;dr on this, I'm sorry