My partner (24M) and I (22F) joined our friends on a trip to a beach to try acid for the first time. There were only 5 of us, two boys and three girls (two couples, one extra girl). We took acid while there was still sunlight because apparently it takes ages to take effect, and we spent this time drinking beer near our tents with a campfire going on. Mind you, as we were descending into tripping balls on acid, we were also getting quite drunk. I'm very lightweight so I didn't have much to drink; my partner on the other hand had plenty. When it started to kick, it was fun. It was funny. I felt like the sand was grassy and I kept hearing Billie Eilish singing to my ear when really it was the wind blowing so hot it was whistling. This is where it goes downhill. Maybe like 4-5 hours later, I decided to go on a walk and had two friends join me because I couldn't go to sleep. My partner wanted to stay in our tent because he wasn't feeling well, so I said sure. Our other male friend who was coming to walk with me told his girlfriend to stay with my bf so that she could look after him, in case anything goes wrong. We thought all was well, until we got back. When we returned, first thing I heard was just obnoxious moaning and breathing. Me and the others panicked so we immediately went into my bf's tent and lo and behold. Both hot, someone's gf on top of my bf. I ended up vomiting and passing out on the shore, but I heard a lot of yelling and crying. I woke up being in my tent and saw my bf sleeping next to me. Honestly, I had to really think whether I hallucinated him cheating or that it really happened.
Our other friend's girlfriend had apparently left during the night without letting any of us know, so there were only 4 of us. My partner's excuse was that he genuinely thought that the girl was me. The smell, the eyes, the hair, etc. When I realised it was all real, I just cried and had been crying since. I've never felt so broken, betrayed, confused, and hurt at the same time. I want to make excuses for him and think maybe it was just a mistake, but he full on cheated. Four years down the drain, just like that. I'd been staying at my sister's since we got back, but he hasn't stopped texting/calling me to apologise and say that he's disgusted of himself and that he genuinely thought it was me. I haven't spoken to him since, but I'm so tempted to. I miss him but I'm just so hurt. I'm so confused. Should I break up with him for it?
EDIT: He'd been planning to propose to me for a month. He apparently made a custom order for an engagement ring to suit what I specifically liked. Told him that he can forget about proposing, as he'd fucked up big time. For context, the night it happened he said that it was so dark in the tent he could barely see anything and that the only light source was the campfire behind the tent. He wasn't wrong in that part, but surely you could feel and hear the difference? The girl had a deeper voice than I did and smelt entirely different from me (we have different perfumes), and she was blonde. I dyed my hair straight ginger. Dropping the proposal bomb on me just left me all the more confused and torn. How am I supposed to break up with him when he had been planning this all along? I feel like the asshole now even when I know I'm not. He knows I'd been waiting for him to propose for over a year now.
UPDATE: I've read all the comments, I am so thankful for everyone's time. I posted this because I honestly don't know what it's like for everyone else to be on acid, and whether it's even possible to confuse someone for someone else because it didn't happen to me. I understand now that he couldn't have possibly confused me with her, but I've seen some comments where he might've been passed out and woken up with the girl straddling him and he just naturally thought it was me (being drunk, high, and in a pitch-black setting) which ultimately is r*pe/sexual assault. I've decided to meet up with him to ask exactly what happened. How it happened. I'll update y'all again tomorrow night. Thank you so so much for all your advice. You've given me more clarity than anything else.