I think, like with alcohol and substances, people become addicted to certain emotional and psychological states -- the good and bad. It sounds horrid, but it's true. A single thought (good/bad) can release a chemical in our body that we become dependent upon. Mix that with an external chemical and it's a toxic relationship that feeds off each other.
Regarding the past... this is also my issue. Grow from it, learn from it, and let it go. Sounds easy, I know! When others have realized that you've moved away from it and have grown from it they can't use it against you.
These are just theories that make sense in my heart/mind. I am learning each day as well. I am trying in earnest to take them from the theoretical phase and apply them directly.
Sigh! A big SIGH!