The rules for mortgage interest and property taxes are slightly different.
You can deduct mortgage interest if you actually pay the interest, and if you are a legal or beneficial owner of the home. You don't have to be on the deed (legal owner) or be a borrower on the mortgage. Without getting into the weeds of what a beneficial owner is, I think you likely qualify. So you could deduct the portion of mortgage interest that you pay, and your partner can deduct the portion that they pay.
However, you can only deduct property taxes if you pay the taxes and you are the legal owner against whom the taxes are assessed. If you are not a legal owner on the deed, you can't deduct the property taxes even if you pay them. (But, assuming you and your fiancé share finances, they can deduct all the property taxes by saying they paid that part of the bill from their funds.). Also note that marriage imparts ownership, so when you get married, you can deduct the property taxes even if the deed is not changed to add your name.
