My boyfriend had sex with his best friend (girl) the minute we broke up?

Me and my boyfriend have been together 3 years. I have had doubts about his best friend who is a girl but i have always trusted him so decided to leave it be. However, we recently broke up and got back together within a week. I found on his phone (I had to use it due to mine being out of battery) that he had gone to her house and they had sex as soon as we broke up. He denied it to the last until I told him I saw the texts and he admitted everything. I have sacrificed so many friendships for this person and he has betrayed me in the worst possible way. Any time I look at him all I can imagine is her and him together? I don't know what to do at this stage, my friends and family hate him and have told me to block him but there's still feelings there and we both have tattoos of each other's name so I feel like we have to be together now which is stupid I know.
Should I forgive him and work things out?
Vote A
Leave him completely.
Vote B
Select age and gender to cast your vote:
+1 y
We both agreed it wasn't working anymore, I've had a gut feeling she has something for him and he admitted while he was having sexual he felt something for her and I left straight away but he followed me home insisting that deep down he doesn't feel anything for her when he thinks about it and that it's me he loves not her?
My boyfriend had sex with his best friend (girl) the minute we broke up?
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