You should do some thorough research on his health conditions.

No one can effectively help you just upon the information you provided. Both spine surgery and depression sound scary because we are all physically and mentally healthy so we have no clue about them.I suggest you search the potential consequence of his surgery. If the surgery didn't resolve his problem perfectly, his disease is being developed badly, what would the worst case scenario look like at the end of the day? It also makes difference if many researchers are actively engaged in fighting against his disease then you can reasonably expect new treatment to help around before it is turning too bad.

Depression varies a lot from one patient to another. Some cases are so serious that patients can't hold up normal life. Some case are mild. Patients can be self sufficient, independent,and even happy.You should get insights into his case before making decision.

I really feel sad you only think about the future of yourself after 2 years being loved and cared about. Selfishness is rooted in the nature of human being, but there is clearly a bottom line one should be careful not to cross if he/she is willing to enjoy the relationship and dynamics with other ones.

Being responsible for your own life is important and wise. But you need to have every piece of information on your table to make a responsible decision. Lack of knowledge certainly cause panic and confusion.