My best friend is dating my ex. Is he still my friend?

So, um, here's the story. He knew this girl before I did, but I started dating her first. Later I broke up with her (not really sure what I was thinking). A few months later I began to regret it, so I called her up, but she told me she was dating my best friend. This made me super jealous and my friend had never mentioned it to me before. I ignored him for a week or so but... I don't have any other friends so now we are hanging out again. I feel like I wish I had some different friends so that I never had to see him again... but I'm really weird so people rarely understand me. So what I'm asking is should I stop hanging out with him and be friendless for who knows how long, or just try not to think about it?