If my girlfriend and I are splitting up after 17 years and she bought a house 12 years ago for which I've paid towards the mortgage, am I entitled to any compensation?
Question Details: I paid roughly $50,000.
No, you are not entitled to any compensation (assuming you are not on the home's title, and you do not mentioned being on it) unless there was a written agreement between you and her that in exchange for you paying/contributing towards the mortgage, you'd be entitled to a share of the equity or other compensation. A person voluntarily paying costs for another does NOT make the other person obligated to repay or compensate them in any way, or give the payor any rights to the other's property, in the absence of an agreement, since you cannot use your own voluntary actions to obligate another person.
Unless your name is on the deed (which you did not indicate), then you are not entitled to any compensation. That is unless there exits a written agreement providing otherwise. The fact is that you voluntarily made the mortgage payments, so you in essence made a gift. Accordingly, you are not entitled to any of the equity or repayment of the money you paid.