How can we live life to the fullest and feel happy every day?

I have become senseless, careless and have avoided life in so many ways. This has happened because I have been negative and pessimist many times in my life. It seems that it has become a habit in my life; a way of being. How can I change this point of view and become more positive? I am tired of seeing everything dark. I am afraid of life, of others and always thinking about all the bad things that could happen to me and my loved ones. I want to stop thinking this way and change my life. Not that I have a bad life, but I have become accustomed to this way of living. How can I change this and stop being negative since it has impacted my life in so many ways? Thanks in advance.