Hi kitkat :)
Sounds like you've emotionally detached from your husband. Sometimes it happens as a result of a choice he's made (affair or some deception), sometimes I think its possible to just fall out of love & once that's happened its hard to get back.
Its OK, it happens. Right firstly, remember that as a single parent you will be eligible for tax credits, child benefit & depending upon your circumstances possibly: housing benefit, income support, social housing. Remember also that your husband will have to provide financially for his children too. Its hard but it can be done.
If you're not happy & you don't want your husband to touch you & you feel trapped then the older children will already know that something's up. If you split then it wouldn't necessarily devastate them as you would be happier by yourself & its better for them to see you happy than not. Also your H will have twigged & staying in a loveless marriage is absolutely no fun for either partner so once the dust has settled he might find he is happier as well. You might also meet somebody else, you might not be on your own forever.... Its a myth that keeping the marriage together for the sake of the children is the right thing to do, it doesn't lead to happier children. The parents being happy leads to happy children, whether that's together or not.
Also you need to think about you, you deserve a healthy sex life & you deserve to be happy & with a man who contributes positively to your life.
(Also although it seems as though your feelings stem from the rough patch 4 years ago, is it possible that you have untreated PND ?)
I know that when you're in a marriage it can seem so daunting to leave, it can seem like so much work but really its not. I left my exH when my 2nd baby was 4 months old & I've not once regretted it. I look back & wonder how I lived like that for so long. I don't think you're stupid or that your choices were stupid, I think you need to live for you now & make the choices that would make you happy :)