Torn - Others (more experienced than I) have waded in on your sitch so I will stick to practical advice re finances.
Like most others here I believe in trying to do what is fair and reasonable. Act with integrity so that down the line, you can look at yourself, and say "I did the right thing".
You need to know what you want and what you want must be such that you can look in the mirror and respect yourself (both because you have been fair and that you have stood up for yourself). Before you can do that you need to get your ducks in a row. Put a spreadsheet together of all your costs. Divide it into mandatory ongoing costs (mortgage, utilities), optional 'chosen' costs (clubs, activities, health insurance for the kids) and shared costs which come up from time to time (e.g school excursions) and start from there. See what you can both afford and see what needs to be flexed.
Before you can do any of that, you need to know what is going out. Don't just blindly keep giving her your checks.
Oh, and if you do put the spreadsheet together, do not show it to her until you have shown in to your lawyer.
