I am an atheist but I believe that religion can help people because, right or wrong, it gives them a purpose in life and I think we all need a purpose to stay motivated.
I think this is the problem you are having, you don't really feel like you are doing anything worth while and that is depressing you, I don't think it really has much to do with your grandma. If you got a pet you would be responsible for it so that might help you or maybe you could help out at a charity or something.
Do oranges know what colour they are?
