What kind of reimbursement is appropriate if I contributed to mortgage payments for my (now-ex) partner's house?

My relationship of 10 years recently ended. We were never married, and were not registered domestic partners with the state of California, although we are registered domestic partners in the city of San Francisco. Three years ago, my partner bought a house in San Francisco the title is in his name only. We left my apartment to move into the house together. I contributed $19,000 to the down payment with the understanding that he would return that sum to me if we ever split up. In addition, for the past 3 years I have paid half of the mortgage/utilities/household expenses each month. I never contributed to property taxes. I have never viewed the property as "my" house, though he always referred to it as "ours." Now that we are separating, I'm the one who must move elsewhere. I'm not sure if it's relevant, but he will almost certainly make an enormous profit when he sells. He has already returned the $19,000 I contributed toward down payment. He says that he will return all of what I paid each month toward the mortgage. However, from that sum, he wants to subtract half of the property taxes he paid for each year I lived in the house, as well as whatever percentage of my mortgage payments went toward paying off the interest on his loan. We have no written contracts (I know...very stupid). I would like to know if what he has proposed sounds reasonable. Is it fair and does it make sense in terms of his financial calculations? I want to be reasonable in my expectations of what he "owes" me, and I don't even know if he does, in fact, owe me anything, legally. But if his true intention is to be fair and decent, does his proposal reflect that? Thank you very much for your help.