another posted alluded to this but I'm going to say it even more strongly: Don't assume he's going to leave S. unless you have some hard evidence things are moving in that direction. Sorry to sound pessimistic, I've just heard too many stories of guys who promised they were going to leave their primary partner and then never followed through.
Take care of yourself and I'm sorry your first poly experience had to be like this. But just think, in a few more months you'll have learned so much that you'll be on this forum giving advice! maybe..??