Confuseddad, thanks for the sympathy!
My marriage is unsalvageable. The scenario you describe is what I fear - pottering around living virtually separate lives, becoming elderly, bored, having missed happiness in life. I also fear DC know things are not right. It is a sensitive age and they are really blossoming right now, well balanced, doing well in all ways. This makes it feel more impossible to split. If they seemed miserable and screwed up it would be easier. I could justify it thinking it might be better for them but I just know it wouldn't.
I really do feel I can't leave, I am doing a course which has 3 years to run. It's very very demanding. I fear I couldn't cope as a SM. DH is 14 years older than me, I find him so dull and middle aged, not that I am irresponsible but he is uber-responsible. We do work well as parents, have the same attitudes to parenting and both love and want the best for the children. He loves me but also this is marriage no2 for him (first one with DC) and he can't face it ending. His business is failing, I don't not care about him. The guilt would be unbearable.
I have other complications with an OM as well, not an affair (wish it was, no sex in my life) but can't say more, I'm too scared someone I know in RL might recognise me.
You need a good reason to split when there are kids involved. Unfortunately for so many people that reason comes in the shape of another person and leads to carnage. I have a sneaking feeling that's what will end up happening in my case, I cannot face growing old with my husband.
This is not my thread, anyway, so enough about me, only you did ask.
niceguy's advice is very good, I think. You need to ask yourself about your feelings above all. Wanting it to work is not enough, everyone wants it to work when there are DC involved. It can only work if the love is still there and enough to build on, and it takes effort. Like niceguy, I think your feelings right now and the state of your relationship are quite normal for first time parents of a 2yo.
I'm curious to know how old you are as well.