My God. That novel could easily have described life with one of my exes. So forgive me for my impending bias:
Get. Out. Now.
I really can't stress enough how important it is to fly the coop when you start noticing patterns like these. Try re-reading your entire post and see how it makes you feel. Got that little sucking feeling in the pit of your chest? There ya go. It's all too familiar, and I can assure you that there are quite a few of us around here who've been down that road before.
Sometimes people just change. There are a variety of reasons why this may have happened. He could have had the time of his life while on vacation and decided he liked the swingin' single life, or he could have simply decided that being paired off wasn't for him and acted accordingly. Either way, it is not your fault, and there's nothing you can do now (or could have done beforehand) that will offer you a degree of control in what has happened. To put it bluntly, there just isn't much left, my dear.
And I'm sorry to hear that. I know it hurts like hell, and I know what I'm about to suggest is going to hurt like hell, but there just isn't any way around it. You're going to have to gather up all the strength you have and leave. It's not going to be easy, but neither is sticking around and hoping for a miracle that will never come.
You've drawn up an entire litany of reasons why the relationship isn't working for you, and it appears the only thing in your favor right now is familiarity itself. All too often we cling to what's familiar because the other option is just so daunting. Fear of the unknown is a great equalizer. It keeps us in places we really shouldn't be, and it makes it hard for us to abandon that safe spot for pastures that are undoubtedly greener. This is one of those cases where you just need to let go of a bad thing to find something better.
Of course it's emotionally messy, and don't think I don't have some of that stuff hanging on after spending a good portion of a five-year relationship hoping things would miraculously get better. Like you, I somehow went from boyfriend to "best friend," and all the signs were exactly as you've pointed out. And so I hung around and hung around and hung around some more, waiting for lightning to strike. It struck alright, but not in the manner I'd expected. Being cheated on did it for me...are you going to wait for that to happen, if it hasn't already?
This person who's been there before advises against doing that. I've walked away from good things and felt far less pain than when I hung around for years and got cheated on. Neither one is palatable, but sometimes you really need to look out for your own emotional well-being. And if that means gettin' the hell out of Dodge before things degenerate further, then that is what you need to do.
Another huge problem many people have with walking away from serious (but failing) relationships is that they have often neglected (whether wittingly or unwittingly) other friends in favor of the all-important significant other. It happened to me and I've seen it happen to others, and the consequences usually don't manifest themselves until the point where they are the most problematic. Basically, finding out you've drifted apart from some close friends becomes many times worse when you make that discovery during the course of a breakup. There you are, needing friends to rely on; when lo and behold you look around and find them off with other people. It's shocking, and that fear alone is enough to prompt many people in bad relationships to stay in those relationships...rather than having to deal with that scary outside world again.
I could put you to sleep with this stuff, I'm sure, but I'll try not to. I guess this is what happens when you finally get to those greener pastures and have a chance to look back upon the wasteland you came from! Go and give yourself that chance. The Air Force teaches you to eject from a stricken airplane and get as far away from the flaming wreckage as you can; and this concept can easily be applied to a doomed relationship. Get out of it, and get as far from it as you can. You don't need the trouble it attracts, and you don't need the damage it can cause. You need something better, you deserve something better, and you can easily find something better.
Take care of yourself, darlin'. And know that at the very least, you've got friends here to take care of you as best we can. Good luck!
BruinDan, "Number Three," PBOM
Beware the naked man who offereth you his pants.