It's not that you're not good enough for him...it's that he's not good enough for you. You have to believe that you deserve better than this...and that there IS better than this...it's so easy for us to feel **** about ourselves (some men have a way of making us feel like that) and to actually convince ourselves that we somehow deserve this treatment...WE DON'T!!! Accepting life with an addict is one thing...accepting life with a cheat is another! Learn to love yourself...forgive yourself...believe in yourself...
I am so sorry for your pain...it will pass...eventually...good luck
xx