Hi Shmmee. I'm a bit of a lurker on the forums in general and have only just stumbled across this thread, and feel I can input as I've been through it in a very similar situation (but without the kids). My ex-wife was fine until her Mum got in her ear and then it all went to shit and the word "fair" lost all meaning. I put her through Uni, paid every bill, including the mortgage on the house (NOT from a joint account) and contributed everything (including the cleaning and cooking, if I'm feeling honest about it all) and paying for the mediators (who were from Cov and very good - referred by CAB) . Now the good news, so to speak.
Weigh up the monetary cost vs the lack of stress. It took me ages and I felt worn down, but in retrospect I should have done it almost instantly. Simply find out what she wants. Get it in writing through somebody legal. Take a long hard look and stop thinking about the time, tears, sweat and stress you put in previously. It's just a number on a piece of paper, and some stuff. Disassociate yourself. Half and half? No problem - ask to be bought out of the house (should be easily done if you've increased the value with repairs and investment) and then MOVE ON!!!!! Every penny you feel aggrieved to give away is ABSOLUTELY WORTH IT to be free of future encumbrances and months of resentment building up. The only thing to be concerned about is your kids. That's it. Get that in writing at the same time to ensure 50/50.
50/50 is the absolute best you can hope for, so don't quibble over objects and numbers. Get it done, and done quickly. Release the burden as soon as you can and rebuild yourself and your life with your kids before it all becomes poisoned (and it WILL if you let it).