Why my best friend hates me because of his girlfriend?

Lets say my best friend is called gareth, and his girlfriend is called courtney. I have been best friends with gareth for two years, and we were really close friends and I found out he liked me and I told him I didnt like him back and it broke his heart, and I’ve apologised so many times and I thought he’d gotten over it. And recently he got together with courtney. And everything was great because he was happy and I was still good friends with him and I started becoming good friends with her. During this time I got together with a guy called ‘josh’. And me and josh were okay for a while and then everything started going downhill and he’d say that we could go out for a night then go out with some other friends without telling me and leaving me at home. He did this again and went to gareth’s house and got drunk and I had no idea, so I called him and asked why he didnt tell me, and my friends started texting him telling him he’d upset me when I was just curious to why he would lie to me. Thats when my social life went down the toilet. One day, courtney was crying for three hours, and I didnt know why and she said to me, “I think you should stop waiting for gareth because he;s going to be a long time and he thinks your a fat ugly cow” and I didnt understand until she explained and said gareth told her that I’d “always be waiting for him” and that I “loved him” when I’m positive I didnt say that, and even if I did I wouldnt mean it, I’ve never liked him that way and I never will, now her and gareth have both fallen out with me and they both hate me. Also, gareth said it was because I always upset kris because I didnt notice him, when they have no idea what was going on in our relationship. But they;re trying to make my life hell by inviting all my friends out and not me because they “hate me and they want me to be lonely”, I have no idea what to do… Any suggestions?