Sounds like you've fallen out of love with your DH. Late arrival from work, whatever the focus to blame it on, lots of people fall out of love. It's sad, but happens. It may help to acknowledge this falling-out-of-love, and accept it as hard as it is, and then move on to whatever happens next, however scary. Would help your DH too, as whoever's on the other side of this type of dilemma deserves to be respected unless they've done something heinous?
It's not admirable or 'moral', but the OM exists, and you have to deal with it. He may be a 'bridge' out of your relationship, or something longer term. You'll need space to work it out, as others have said. It's sad that people think because he took on another child,then separated, that makes him odd and deserving of a 
. Long term relationship from the sound of it, where he took responsibility for being in another child's life. And still is.
Life is never black and white, nor feelings. I'm a very, very long-term lurker, occasional poster, but felt I had to reply, as you've been judged very harshly when you're using an anonymous site to voice the tensions in your head you're obviously facing. There's probably other reasons your relationship has broken down, in terms of communication and love, which you can rationalise into blaming 'his work'. I was in your place a long time ago, without the distraction of an OM. But I do wonder if an OM had been put in my sightline, whether I'd have seen it as an escape pod. The escape finally occured without the need for an OM, and life is much sweeter years on (though it's been years to get there!).
And I'm wondering if we refer everyone to Riven's difficulties every time someone posts a 'trivial' post, then there would be no more 'plate stuck down the back of the toilet' threads, would there? Surely the site reflects all aspects of life.
Essay over!