One of the first things that stood out for me is that you're 26 and have been in this relationship for 10 years! I found that about the same time in my relationship (hubby and I have been together since I was 17) that things really got shook up. Although we went in a different direction, from having a mostly closed relationship to opening up. Maybe the shake up in a relationship is something that happens when you get to that point of really being an adult and start to think more about what YOU want.
I think that all of you should sit down and talk about what you want and what your goals in your relationship(s) are. I hope that you can all work out something that is going to work out for all of you. At 10 years in it's not going to be without a lot of emotional involvement. On the bright side there is also that solid foundation of caring and respect that has been built over the past decade.
Everything will be ok in the end. If it's not ok it's not the end.