My boyfriend cheated on me?

I’ve been with my (perhaps soon to be ex) boyfriend for 2 years now and I found out yesterday (which was my birthday no less) that he was with another girl and she was kissing him on the cheek with his arms wrapped around her. It was a couples Halloween costume they did where they dressed up as the king and queen of spades. So when I addressed him about it, he said that it was a joke that his friend made him do and that the girl is his friend’s girlfriend. However, he never told me it happened and I had to find out through Instagram. I’m debating whether or not to end the relationship especially since it’s long distance. Advice?

Update:

We’re taking a break. I’m still considering to end it but I’m so confused. I feel so alone right now. I have great friends who have been nothing but the best support system, yet I feel like I need comfort.

Update pt 2:

we ended it :/ but it’s good because now we get to work on ourselves