I've been married since I was 19, and have 2 primary school aged children.
Dh is a decent enough man, but he is pretty rubbish as a husband. He's hurt me very deeply several times over the years, and each time I stayed in the relationship because I have always been utterly besotted with him and could never imagine my life without him (especially once we had children).
However, once my youngest child went to school, I gradually started to detach from dh for the first time in my adult life. I started finding time spent not with him more enjoyable than the time I did spend with him. It was a totally alien feeling, but I loved it. I finally felt free. None of his selfishness or thoughtlessness could hurt me anymore, because I was finally getting to a point where it didn't matter to me.
It was around this time that I met someone else. Someone who is so so different to dh in every way. We've been 'together' now for nearly 4 years.
When it first started, I had NO intention of leaving dh whatsoever. My life was quite nice, and new man, whilst lovely, was just my way of feeling better about myself after all the years of being let down by dh.
But it didn't turn out like that. New man is everything that dh has never been, and I feel more loved by him than I ever have by dh. He adores me, he doesn't need to tell me - I just know, and I've never felt that before.
He will also ALWAYS make his children his absolute top priority in everything. Providing them with a stable family background is very important to him. At first I was glad of this, as I felt equally determined to do the same for my children. Our relationship was conducted entirely separately to family life, and that was just fine.
It's been so long now though, I'm starting to feel that everyone in this mess is living a lie, and that we are now robbing our current spouses of a fairly significant chunk of their lives 
I'm possibly ready to start thinking about leaving, but I very much doubt that he will even consider it.
Cannot believe that bit by bit, this is where my life has ended up.