If you're the one who ended the relationship, consider being the one who steps up and sorts all this stuff out.
They'll desperately want to do the right thing and not take sides.
They were, and are, great, but I quickly realised that they had a duty of care to my ex first.
I could feel in every text the undertone: "I want to be there for you, but he needs me too." I'd still see our mutual friends, but if I knew my ex was at a loose end, I'd back off and let him be with them. There's a lot to be said for "out of sight, out of mind".
Whether it's fair or not, your mutual mates are going to be there for the one who got dumped first and foremost – that's just the way it is. In your head, you may have been long over it before you even made the move to split, but that doesn't mean you have to rub your ex's nose in it.
At first, I felt left out, but I tried not to let it get to me; I knew I needed to wait things out. I didn't want to force my friends to make the awkward decision on who to invite and who to leave out, so I'd make it for them. Not because I was bitter and wanted to be conspicuous by my absence, but because I wanted my ex to have a good time without worrying I'd turn up.