If you want to believe it, more power to you, but there is nothing selfish about wanting to die. Suffering for no reason whatsoever for who knows how many years is nothing to be proud about. Selflessness doesn't exist, there is always a reason behind every action; We are always looking for something. If you choose to accept that suicide is selfish, then having to stay around for the sake of others is about as selfish. Then again, selfishness is a core aspect of human nature.
Anyways, if you kill yourself, you will not have to deal with any guilt. You will be dead.
I can relate to worrying about those who stay behind, but at the end of the day, you have to make a choice. Very likely someone will end up hurt, whether it is your or your father is the question here. Be aware that even if you stay around, your father still has depression, so it is not like his life will be full of sunshine (unless he finds a way out of it, which would be great, of course).
I guess the fact my father is a nobody in my life and my mother has hurt me so much turned out to not be that bad of a thing, since I feel absolutely no guilty about leaving them behind. Though, on the other hand, I can't bring myself to leave my dog. Well, at least dogs don't live that much, I suppose.