QR - I've hopped over here from the AN club as I was worried about you. I haven't had much of a chance to read everything as DS or DD have been demanding so I apologise if any of this has been said before...
Would it be a possibility, in the short term (next few months, say until the new year) to live together as flatmates? If you generally don't hate each other and have no inclination to finding someone new then maybe a few months, with no relationship, would be a good way to see things more clearly. I'm not suggesting it would make you change your mind, but it might help the practicalities (where to live, how to explain to your DS) and it also might help your DP get used to the idea.
It sounds as if you're feeling really guilty about rocking the boat, but don't. If in needed to be said, it needed to be said and there is no point living a lie. But if you try to think about a few months down the line, in your own place with your DS what do you see? Are you happy? Happier? Are you feeling guilty? Are you feeling relieved but still feeling down?
I suspect that you may be depressed in general and see this as one thing you can make a fairly easy fix on. Are there other things bothering you?
My relationship with DH is under real strain at the moment, but we've both acknowledged that having 2 kids in the house (one who still doesn't sleep at 7 months) makes it a really 'artificial' environment for 'us' as a couple. We don't have a sexual relationship at the moment and it's been sparse for a while now. We don't even share the same bed (I share mine with DD). I guess what I'm saying is that if there are other things causing problems other than your relationship, it can be very easy to blame your relationship for everything. Hope you get what I mean!
Please do take things slowly whatever you do. You've been together 14 years. To split up in 14 days wouldn't seem fitting. Give all of you a little bit of time to come to terms with it. And if you think that there is any possibility of salvaging anything, please do get some counselling or outside help.