daddysgirl, the feelings you describe are entirely understandable - you are trying to deal with something that is inherently horrible and it isn't hard to understand why you believe that you have nothing to live for...
But, simply, IT ISN'T TRUE.
At this moment in time, you are watching your father "wither" as you put it. But what you are watching is not the sum total of your father's life. Your father's life has meaning far beyond the current moment or his current condition. And part of that meaning is you. You are 15 - you have difficulty seeing beyond this horrible time but please believe me when I say that there is something beyond now, for you and for your father.
You may never have heard of Viktor Frankl or his book,
Man's Search for Meaning but it is basically a book about how to find meaning and something positive in circumstances which seem only awful. I am not talking about "finding the silver lining in the cloud" here; there is no silver lining in your father's illness. What I mean, and what Dr. Frankl talks about, is
forcing something meaningful and worthwhile out of circumstances such as the one you are facing. Sometimes, that can be as simple as building a sort of living monument to what your father's life means to you and to other people who know him - your father's
life, not his illness - his life and his legacy is much more than his illness. And ultimately, you are part of that monument and that legacy.
You indicate that you are on antidepressant medication - you don't say whether you are in therapy or seeing a counsellor. If not, I strongly urge you to ask for help in finding someone... with what you are facing, medication isn't enough, as I guess you are discovering. You do not need to face all this alone and I urge you not to try...
Please post again,
daddysgirl, and let us know how you are doing... coming here was an important step for you - don't stop there...