I'm in a similar spot, just earlier in the process. I've been with my girlfriend for 7 years this summer, and all signs are pointing toward breaking up except her determination to support me through this.
She hasn't gone through a major breakup before, (major being such a long term relationship) and she's just not the sort that would initiate it. She is the type that would bottle it up for the next decade if she had to in order to keep what we have, and even if she got an inkling that she deserved a more fulfilling life, she wouldn't necessarily be the one to take that step. So I'm thinking for two right now.
I hear you on the best friend roommates thing. Ever since my breakdown a year ago, I've been drifting in that direction emotionally, even if it wasn't until earlier this year that I realized it was because of my need to transition. We've been attached at the hip for so many years, and it's such an amazingly comfortable and fulfilling relationship outside of all this that I could easily imagine being old together, but with most of our lives ahead of us, it's hard to say that growing old together is the best.
Then I think of how much I need privacy right now, and how uncomfortable I am even with her seeing me while I am going through this, and it just makes me wonder.
And at the same time, I love her and I can't imagine hurting her even if it's necessary.