Is it wrong to leave a mostly-ok 10-year relationship because you've found someone new that you're passionate about?
I suppose this scenario is pretty clichéd. I'm in a 10-year (not married, no kids, but share a mortgage) relationship that's generally fine, but about which I've (mostly privately) had some doubts from time to time. A few years ago I met another woman who has become a very close friend. That friendship has reached a point where both of us recognize the need for it to become something more, or for it to completely end. I feel more connected and attracted to this friend than I have to anyone else, ever.
Some background about me: I'm quite shy, and have had only two real relationships. In fact, though I'm in my mid-30s, I've really barely dated at all. I've had lots of crushes/more-than-crushes, but none of them ever turned into a relationship, partially because I was too reserved to do anything about it.
A few points about my current relationship: As I said, we've been together for about 10 years; lived together for around seven. Five years ago we bought a house together. This relationship is, for the most part, fine; we generally get along well, and there haven't been a lot of ups and downs. We've made a comfortable life together and have been pretty happy. However, if I'm honest with myself, I have a hard time feeling passionate about it, and that feels wrong to me. My partner definitely wants to get married. I've never been comfortable with the idea of getting married, not wanting to just do something because it's expected or socially normal. Lately, though, I've begun to wonder if I'm really just not comfortable with the idea of getting married to her. Part of me feels that if getting married to my partner was the right thing for us, I'd know it in my gut, and that my lack of this feeling is something I should pay attention to. So far I've deferred any decision on marriage, but I've left the door open (to preserve the peace), and I won't be able to put it off forever. (I know, leaving the door open may have been lame, but I really wasn't sure either. I realize now that if I'd dealt with some of these issues properly earlier I wouldn't be in this pickle. Sigh.)
A few points about my friend/more-than-friend: Wow. I'm barely able to grasp how fantastic this situation is. For years we've spent significant (and increasing) amounts of time together during the week, sharing walks and lunches, talking all the time. She's amazing to me in so many ways: she's quirkily funny (matching my sense of humor almost exactly), she's adventurous, she's incredibly smart, she's absolutely beautiful. (There's that feeling in my gut... definitely getting it now...) I find that I'm more physically and emotionally attracted to this woman than I've ever been to any other person. Sparks fly whenever we're together. I've been sort of nursing a crush for probably a couple of years now, but even though we're sometimes flirty, I never knew if she was just being playful or if she felt something more. She's also in a relationship -- been dating a guy for a few years now.
What's happening now: my friend and I have become so close that barely a day goes by when we don't have some contact, be it in person, email, text, etc. We both (and especially she) realized that we couldn't really continue like this -- it was negatively impacting our other relationships. For the last month, we've been trying to figure out where to go. We've had some emotionally difficult but completely open and honest conversations in the last couple of weeks, and we've each learned that the other was feeling pretty much exactly the same way all along. We're mad about each other, and we both desperately want to be together. We're both just absolutely floored at the mutuality of these feelings, and by the opportunity we have to be in a relationship where both parties are truly passionate about the other. We can hardly contain our excitement about our good fortune.
So back to my original question. I, obviously, very much want to start taking the steps to make this new relationship happen. How can I do this while minimizing the collateral damage? I know that my current partner's going to be blindsided by this. I suspect she can detect that there's something going on in my head that's been making me a little distant from time to time, but this news will definitely hurt her. Nearly all of my friends are really "our" friends, and I imagine they'll all be pretty shocked and disappointed too. Is there any way to do this without being a complete jerk? While I realize that not dealing with issues in my current relationship for so long was definitely a mistake, wouldn't it be worse to continue that way?
Breaking up is definitely going to be a mess; I'm pretty terrified about how to handle the house and the mortgage. I know the emotional fallout's going to be intense, and I expect that I'll end up becoming estranged from lots of friends too. But, I can't bring myself to walk away from this amazing chance at the relationship that I've always dreamed about.
So, hive mind, how do I do this? Please give me advice on whether or not I'm doing the right thing, and how I should proceed.