I'm going to break up with my girlfriend after bonding with her 6 year old son
Why do you want to breakup with her?
I'm not good enough or worthy enough for her.
You shouldn't bond with your girlfriend's son unless the relationship is serious. Don't undersell yourself. Your girlfriend seems to think otherwise. Besides, young kids are impressionable and you leaving for no discernable reason could cause the child to have abandonment issues.
I need to be responsible for my actions. I don't want to be a bad influence to them.
It's great that you decide to pull out before things get truly serious, because her son might get too attached to you. I'm sure they will understand that you are just trying to protect them!