Like most post I'll start with some backstory, I'll try and keep it as close to the truth as poss (find some posts here seem like very skewed versions of the truth). 

At the time I had just finished university and was in the stage of trying to stay in contact with my close friends from then. With one friend, let's call her eve, we had been through alot of ups and down. I used to have a thing for her, sheet tonnes of mixed signals where she's say she didn't want anything like that with me, but would then get really drunk and try to sleep with me. I never did because of how drunk she was, definitely in the blurred lines of what could be rape, plus sober she didn't want anything so I respected that. Through the last year I tried to move on, and got a girlfriend, and was happy. It was obvious to see eve was not happy with this, I was open with my gf at the time and told her about the now distant feelings I had for eve. Eve would then carry on to try and break me and the gf up or that's what it felt like. (Won't go into detail trying to make post as short as poss). Gf would get jealous that I was spending so much time with eve (not out of choice we were housemates). After the breakup I would continue to get very mixed signals even though now eve had, and still has a bf. Then on the last night out of university we end up hooking up (I was very intoxicated at the time had dropped some mdma, eve didnt).

After the night I felt pretty guilty as I knew she had a bf but still went there. But I didn't think it was my place to say anything, I don't want to be the reason a relationship breaks up. After uni finished contact was strange very hot and cold. Fast forward a few months to now, eve invited some old friends up for her b day, I wasn't sure if I wanted to go as things seemed very off with her, from some other mates I was pursuaded into going. The plan for the b day was pretty chill, go to a few pubs and bars in the day maybe go clubbing at night. Some of us pre drank before, without eve, we were all sorta dreading it because things between her and us seemed awkward (not just because of me, the others had reasons too). As we pre drank we turned up late which she wasn't happy about, and about two pubs in I was too drunk, rookie error of not eating, so ended up spewing in the toilets for a good while, when I get back I get in a spat with eve because of the state I'm in. I promptly decide to leave.

Later I get a text from eve saying I ruined her bday. At this point I have lost all patients with her and tell her I do not want a friendship with her anymore, the confusion she has caused over the years has left its marks. She doesn't take that well and goes on to bitch to our whole group. 

Since we have not had contact I am a much happier person. But AITA for ruining her bday and the friendship? I can't help but feel some sort of guilt for the day.