How do I stop being hurt and obsessing about my ex post-break up?

I am recently out of a long-term relationship and I have been pretty devastated since. I have been struggling to sleep, eat etc. I also have become more obsessive about my ex since the break-up. Monitoring social media, texting the whole time etc. We met up a few times and thought had gotten closure, but then something else would happen and I would reach out again and a constant cycle would happen. He said he is open to the future and doesn't know what will happen, but not trying to get back together right now. Anyway, I leave the country and a "friend" texts me that he has heard my ex has been hooking up with multiple guys recently and is now very active on the gay scene. I felt my stomach wrench, I immediately text him, he claimed it's a lie, that he hasn't done anything. Just feeling really lost as not too sure what to do. Do I believe him? Or does it matter? Just struggling a lot.