Glad you're still on here jmg - and I have to agree with ff that anything is worth a shot, including counselling. I know why you don't like the idea but even if you don't feel like you can rely on someone else - don't rely on them, just use them as a stepping stone to you getting your head back together by yourself.
And sorry to get all know-it-all but you're being FAR too hard on yourself. It might not sound like much but after everything you've been through, but just getting yourself out of bed in the morning and bringing up three of the most gorgeous, funny, well-behaved kids around is a HUGE achievement. Forget being focused on anything else for the minute - the business is important but its not the be all and end all in your life. If you lose focus on that for a while and you can't put as much into it as you used to, it really doesn't matter - its NOT a failure on your side.
Medication may help you or it might not - with the range of stuff available now it would probably be worth a trip to the gp but that's up to you. Talking things over, IMO, would definitely help. It doesn't even have to be a proper counsellor, at least to start with. Just spout off to anyone who'll listen (and don't worry about whether you're wasting their time - that's their problem and they're probably happy to listen anyway). Then you could think about a counsellor - just try it and if you hate it you don't have to go back (just make sure you pick a qualified one - there's some out there with no experience that do sod-all for you). But keeping things to yourself obviously isn't solving everything so talking to someone else might just be worth a shot.
But its really important to go easy on yourself - you're a fantastic dad which is hard even if the rest of your life is perfect. There's nothing wrong with hitting rock bottom - its no reflection on your capabilities, and very slowly things will start to pick up. It will probably take years and be painfully slow but it will happen.
And for what its worth, your kids seem to know they can rely on you and trust you totally.