I've come across numerous situations like this among friends, OP I feel very sorry for you and hope you can find a way to work things out.
Not to generalize too much, but from what I have observed, the relationship initially seems fine; children come along and boom a couple of years down the line it's in tatters. The overwhelmingly common factor in these situations seems to be that the female partner decides to bail and the male partner is left asking questions.
A good friend of mine is going through something very similar; I hope by sharing his story I can at least offer some form of support and to reassure you that this isn't uncommon, and it isn't all your fault.
He's a good provider, an excellent father and a supportive and attentive husband. She has now told him that she "misses the spark" they used to have and now "wants space" to "find herself again", which has culminated in her moving out; they are currently trying to work out a shared custody plan of their three year-old. He is shattered. Never saw it coming. Was doing his very best to provide for his family and this has knocked him for six. She accuses him of being "too serious" (he is a serious person, is successful by dint of hard work and taking things seriously, and uses this success to provide for his family) and that he's "boring and predictable" (maybe, but do you really want to be married to someone who is unreliable?)
I don't know why, but maybe she has an unrealistic view of what life is like after children come along. Life isn't champagne and chocolates every day; it isn't like it is in trashy chick lit and certainly isn't anything like the innumerable romcoms that poison the screens.
In the initial throws of a relationship there is of course a spark, things are always fun and the other person can do no wrong - however a few years down the line when you are familiar with each other and the bins need to go out, the dog needs walking, the bills need paying, the dishes need doing and the life insurance policy needs to be sorted out - general mundane everyday things that life throws in your way - the spark can turn into something else. Sadly I think too many people can't handle this and choose to throw away their relationship rather than work at it