Thanks so much. When we moved in, We agreed on a contribution, and I'd do food shopping. Its been working ok, We haven't argued about it. He's a brilliant lovely partner and is really generous. I'm just noticing that my portion 'left over' is significantly less. Many people would think this is right - after all he didn't father the kids!
Hang on a minute.... he may not be their biological father but he's now part of the family and responsible for them just as much as you are. The aim should be to have a similar amount of personal 'spends' each and the remainder either goes to paying household bills (including any specific costs relating to the DCs) or into family savings.
I agree with Cogito, if you're living together you need to be thinking in terms of family finances, not separate pots split by salary/whether they are the dc's parent. I already had dcs of my own when I married DH, and was in a position where I was out of work shortly after we married. I lost tax credits and all other benefits except disability benefits, and of course DH supported us all during that period. He wouldn't have considered any other option, and there's no need to feel guilty because your DP has obviously chosen to live with you and to take on that responsibility.
The DWP/HMRC expect him to pay up as well, that's why they take his income into account for tax credits, and they aren't interested in whether he's the biological father or not! I don't see how else it can work otherwise - what if you couldn't work for a while? - the state wouldn't step in to help you, because they expect the family finances to be shared. And you can't cover all your dc's expenses by yourself because ultimately they'll lose out, as you've lost your tax credits. What if you have more dc together? Would your dc end up being treated differently than his biological children?
If you've moved into his property then you also need to consider longer term security for the dcs if the relationship ends/if he dies. With DH, I decided not to live together until we were married, as marriage gives us lots of legal/financial securities that cohabiting doesn't. If it's his home, he'd be entitled to ask you to leave at any time and that can't be very secure for you.
"I have a real problem with money and attitudes towards it. I worry constantly about it. "
IME it's more worrying if no-one says anything and the finances are too vague. If you can have a mature & open conversation about the whole thing and set up the various accounts and direct debits etc., I think you'll be more reassured.
If you're nervous about having joint accounts (and I don't particularly like them either for the same reason as you... a shit XH) then you could set up one joint account for all the household payments, a joint savings account for things like family holidays, house repairs, replacing appliances.... and keep your own accounts for everything else. All that's important is that it's fair, mutually agreed and treats your DP as part of the family