Double date

So tonight I'm going on a double date with my ex and his new girlfriend. I'm going with one of his friends. Me and my ex haven't seen each other in over a month so I feel like it's going to be kind of weird finally seeing him but at the same time happy. Idk if I should do this…is it too soon? Should I back out? I'm just really anxious and idk what to do. Please help me. Just any advice or tips on what I should do.