Am I a horrible person?

So my boyfriend broke up with me about a week ago, he said it was because we were having silly petty arguments. He told me he wanted to be single and go have fun and I should to and maybe in the future we could fix things.

Last night I made the biggest mistake of my life, I got extremely drunk and slept with someone else, I told my ex and now he wants nothing to do with me ever.

He said it counts as cheating cause there was a chance we would get back together. He said I’m a horrible person and a slut, am I? I’m starting to think I am. :(