lvd, why would that make you think that you are not good for anyone?
Being in a relationship with an addict of any kind is a thoroughly trying experience, it's not like a "normal" relationship, it's not like the person is reliable, generally, or dependable, so it's not surprising that might make you feel insecure.
Has he ever given you a reason to mistrust that he is faithful to you? Do you just not trust him in general anymore?
I noticed in the other forum that someone recommended Codependent no More, by Melody Beattie. It's not suggested because anyone is saying you are codependent or not, but it's got great information about relationships and boundaries that you might find really helpful and reassuring, it can help give you direction and question your perspective on things.
Hang in there, this can't be easy. I really do second the idea of going to Al-Anon, or even AA meetings that are deemed as "open" meetings (ie: you don't need to be an alcoholic to attend).
It sounds like perhaps this relationship has diminished your self worth and self esteem?