fwiw, I think you sound a wonderful woman, who will always take superb care of your DCs.
You've described your H as selfish and say you doubt whether he'd do the best for your children if you split. While I realise these factors don't make him a bad man, or a bad dad, I wonder whether his presence in your family really is such a positive overall influence. Coupled with your dull un-happiness, their experience of family life might actually be rather flat & grim. Again not an especially bad thing but, children being so responsive to their emotional environment, not the best one could hope for.
It's brave of you to acknowledge and face your issue. My own feeling (which has changed as I grew older & more experienced) is that your happiness, as the main child carer, has a tremendous impact on your children's wellbeing. This could read like a apology for 'disposable marriages' but, obviously, that's not your case. You've tried everything you're supposed to try.
The main reason behind "staying together for the children" is social conformity. When you think about it, though, the 'traditional' nuclear family is no longer the norm. Our society no longer requires (or even expects) it. What advantages, then, do you bestow on your DCs by staying together? You are probably better-off financially. You need to weigh that up. How much happiness and FUN does a skiing holiday, or a bigger trampoline, add up to?
I'm sorry this is a long reply! I thought maybe you'd want to chew your question over in a bit more depth ...?