Thanks for your replies.
The problem is that Stephen says even if I do break up with Chris we won't automatically get back together, he's not in the right mental place for it. But he said there could be scope for us in the future.
I think being with Stephen has opened my eyes to some ways I'm not satisfied with Chris. I still love Chris really deeply though and can't imagine ending it with him - it would absolutely destroy him
There's also no chance for the three of us being together again.