Sad title to a thread. But I could write that myself sometimes.
I'm out the other side, separated about a year ago. We actually agreed to separate before we physically did (took a while to sell the house). What was very telling was that having agreed to separate materially our relationship stayed the same, albeit more tension and resentment from dh who didn't want to part. But really not much changed.
OP and triathlon your marriages sound like mine was. I'm not flying the flag for separation, it's been horrible, really horrible. But a year on I am happier. My own boss. The guilt is diminishing. I had two dc a little older than yours. They adapt - they have to. I'm glad we are no longer modelling a bad relationship.
The worst bit is where you are: stuck in an impasse, a deeply unfulfilling relationship that you cannot bear to leave. You can only leave when you are ready.
I'm not saying this is necessarily inevitable for you, but if someone won't meet you halfway then things can't change. If this is the case eventually you will decide that you have to make the most of the years you have left and get out.
It's an old cliche but being lonely in relationship is worse than being lonely alone.
I wonder how old you are... I'm around 40. I missed a Uni reunion recently, partly because I couldn't face telling people I'd separated. The last one was seven years ago and only one person was divorced and it was 
I got reports from a friend about the recent one. I asked if anyone was separated/divorced and he said about a third of people.
Heard over the last couple of days from two male friends who have just left their wives, not for other women. Both... around 40. Marriages toppling like dominoes.
Once you get to mid-30s/early 40s you have made a life for yourself. You either like it and want it to stay the same or you don't and you have to make changes.
Dealing with separation is a whole other thread. But I don't think the marriages here can endure unless the other party is prepared to change. Even then it may be too little, too late.
I'm sorry for anyone enduring this pain, but eventually you will have to bite the bullet rather than condemn yourself to more years of misery.
I only wish I had got out sooner. I am lonely, very lonely, sometimes.
But no longer all the time :)