When is it time to get divorced?

I don't know how to say that i don't want to be married anymore. When we met (12 years ago) he was bright and full of life, ambitious and had so much depth. But, he has Schizophrenia and Severe bipolar. for the past few years there has been a steady and rapid deterioration of his general state of being and he has gone from being this magnificently capable man to a a child in so many ways. I have to remind him to eat, shower, change his clothes, etc... He can not work, he can barely function on a basic level at home. The smallest tasks(washing dishes, remembering to let the dog back in the house..) overwhelm him and he essentially needs to be coached through them. I work two full time jobs, though one I do from home. I went from being independent and happy and feeling like I had numerous opportunities in life to feeling like a prisoner to my husbands sickness. I care deeply for him and his well being, but I'm not in love with him. But I feel obligated to stay in a sense because i don't know what would happen to him if I left. But I am still very young-late twenties. Our plan has been to travel and see the world, to accomplish so many things that he & I had wanted for so long. But none of that is achievable now. I feel such a profound loss for my life. Like all of my dreams are slipping from my grasp and I am becoming so resentful towards him. I am angry, even though I know that this is not his fault, that I am losing myself to this relationship. But I can't seem to come to a clear decision about what I should do. I feel like a terrible person for desperately wanting to walk away. But I don't know how much longer I can take this burden. This is not what I was meant for, martyrdom. I am not a religious person and I really hope that those who reply can understand that advise from a religious perspective won't really be useful. I need suggestions, help, something...because I am very confused right now