Breakups

A couple months ago my boyfriend and I broke up. He was my first real relationship. I put forward the question of if we were right for each other or not because he seemed to be losing interest and partying a lot. We mutually decided to breakup but it was a very difficult decision and (I thought) we both still loved each other a lot. We kept in touch until all of a sudden he completely started ignoring me after he took the initiative to say that he potentially wanted a relationship again in the future. I was confused but thought we’d still connect someday. A week later he got with another girl and they’ve been together ever since. Was I wrong to be upset about that? Was it ‘my fault’?