So common after treatment to feel like you. The advise already given is so true. For me I think no matter what age we are, until something really goes wrong with someone close (or yourself) we still have that almost childish view that it won't happen to me -- and when it does its a real shock!! You lose your innocence (for want of a better word). Before I was a happy person, now I have to work at being happy. Saying that I think the happy is better when you work for it.
Previously I was bullet proof - now I know I'm not. Maybe the unhappy people in the world are the people who have lost that innocence too early in life....