Hello lovely (and Rebecca2014) - what a dreadful thing to experience. Firstly, no matter what your little voice in your head says, may I be the first to say: You did not deserve this.
YvyB has given you some great pragmatic advice. As much as you can, in the first few days, when the adrenaline is in best supply, crack on with as much as you can (I note you've already done a great deal of work - well done).
Yes, you feel like you're sinking. It's absolutely no wonder. Do not let yourself feel bad about that. It's perfectly understandable
You will be okay in the long term. In the short term, you're going to be all over the place: angry, sad, in denial, devastated and most of all in the first few minutes/hours/days/weeks: panic stricken.
You're not going mad. You're a woman under immense pressure, and has the added worry of looking after three (one) young child/ren.
You're amazing, managing that. The proof is in the pudding: he couldn't/wouldn't/didn't even begin to cope.
As bad as you feel right now - you're still doing great. Come here, cry your heart out, rage but most of all: he's past tense. No begging/raging emails/texts.
Other pragmatic advice: You don't state what your husband earns/whether you own your own home, etc, etc. As soon as you can, get yourself a quick (and usually free) solicitor's appointment. This does not mean you are getting a divorce, but it is the best tonic for understanding what the future holds financially.
For me personally, that was the best tonic in reducing the anxiety of the situation.
Finally, if you haven't told everyone you know already - tell them now. Parents, friends, acquaintances - the lot. It is the kindness of others, not those you expect support from, that is truly amazing in all this.
Take care, keep posting. You're not alone - and you're doing okay.