Advice Please

So on Sunday night (oct 2) my boyfriend and I went to one of his mates parties. There was this girl there who was really nice and me and her got along really well so did my boyfriend. She was telling us about herself talking about her bucket list and one of the things on there was to have sex with a white boy. And I jokingly said "hey just go with him" and pointed to my boyfriend and my boyfriend laughed and then she said "no I wouldn't go there you two are too cute". And then I said to my boyfriend "this is drunk me is saying that, you'll have to check back with normal me tomorrow". So my fast forward to the end of the night and I get dropped by my boyfriend in the middle of town so my mum can pick me up and the girl was in the front seat with him on the way back to the party. I was a little bit suspicious about it but because I was drunk I didn't think much of it. I wake up the next day to find out they made out with each other and I was balling my eyes out and sad that he would do this. When I was talking to him he kept saying "but you told me too" and "she wasn't even that good I still love you". But then he said "maybe we should talk a weeks break just to re evaluate things?" And I said to him "so your not even going to fight for us, for this relationship?" And he hasn't replied. Honestly I just don't know what to do. 
     Side note*
We've been dating for a year in November 
He was my first but I wasn't his 
this is the second time he has done it (the first time was when he had to cut things off with another girl to be with me)