Oh no what do I do?

So it's taken months and months to move on from my ex boyfriend after a long complicated break up. He decided to break contact from me on my birthday and we completely stopped talking. I met a new guy who is wonderful and we were progressing wonderfully until my ex and I both made it into an intense music group together which will force us to be together for 2-6 hours a day... All of a sudden my ex reaches out to me again and we start talking over text once again but I keep trying to keep the conversation brief because I don't want to fall back to him but I can't help it... What do I do?