Big hugs to you, Rach, cause it doesn't sound easy.  Sounds like you and your partner could need some time together (child-free) to give you the chance to explain to him how this all feels for you and what it is that you see happening for Jack.  If you have time beforehand to ponder over and sort out what you want to say, it's more likely to come out how you mean it.  But I guess that only really works if he's willing to listen and not yell at you!  Did he ever have counselling for depression?  I know most men would rather get a fork in the eye than go to a counsellor, but if he's the rare, rare kind who can suck it up enough to see someone, would it help if he went back (or started?).  Dammit if only they could be made to do what  they've sometimes just gotta do, eh! 

Hey I might seem like the devil for suggesting it... but how long have you felt trapped and just how miserable are things at home?  Because sometimes a girl's gotta do what's right for herself and her kids, even if it might not be the 'right' thing for her relationship.  I guess if there were good times in your history together then it makes sense to expect that things can get better again (and here's hoping!), but if you don't have memories of when things were better, that's a different story and makes it blimmin hard to work through the problems.  I rekon talking to your friends would help - might be a bit embarrassing at first but some support from people who know the both of you could be really nice for ya.