I just found out that I was a unicorn

So to start off I (F21)mono-straight, started dating this guy(M22) in 2018 we met in high school and he was so romantic and charming until he got locked up for a year when he got out were on and off a lot towards the end of our relationship and really just became friends with benefits I was loyal to him for the longest ever in our fwb situation he knew how much I love him and he took advantage of it . One night he wouldn’t answer the phone and I felt sick to my stomach I just knew he was cheating so I pulled up to his house he was there but he would open the door I cried at his window for a good 10 mins but decided to just leave the next day I felt crazy bc he made me believe I was ‘trippin’ and I felt like a psycho. Fast forward to around Christmas of 2022 we still been seeing each other for the past few months but he has been very distant and busy and one night he texted me saying “I miss you. I wanna talk about our future, I want to take of you. Come move in with me” and I’m so excited I been waiting so long for him to tell me this. So he picks me up and we get to his place and I go in his bedroom and there’s a girl(19F) sitting on his bed so he starts saying “look just hear us out, everything is fine, just give it a chance” at this moment I have so many questions and feelings I can’t even fathom what was happening . I looked at her and said wtf is going on and who are u she said “I’m his girlfriend what do u mean?” I just smiled at him hatefully and everything made sense now all the dots are connected in that very moment, so I couldn’t trip out on him bc me and him weren’t together at the time so I am in there talking with them. I’m surprisingly calm. I kinda wanted to see this through now I regret I stayed I should’ve left the moment she said they were together but I was in love with him. So I tried the “trouple” thing and I swear every jealous bone in my body ignited and I hid it for him. Come to find out that she was the girl there that night that I was crying outside his window and I asked her about it and she said that she remembered that happening so im thinking like so he cheated on me with you and now you’re over here dating him and I’m the unicorn and this is after I had sex with both of them and that literally killed me and I just can’t believe that I did that I did it a few times with them. I stayed with them for about three weeks but when I found out he was cheating with her I couldn’t take it anymore and I told her just bring me home. I’m so fucked up behind all of this and I feel like it’s my own fault I just need someone to talk to.