Am I sane?

I am the wife of an ADHD spouse.  He was diagnosed about 3 years ago and started taking Adderall at that time.  About 4 months ago I told him I wanted a separation after 10 years of being together, 7 married and 3 before that.  The thing is, he is taking medication and now he wants to get counseling, yet I wanted to get marriage counseling 3 years ago.  He turned me down and got angry because nothing was wrong with him.  We have a small child, 4 years old.  I guess I'm just to the point that I don't want to try anymore.  The only reason I think I would stay with him is because I know that he truly loves me and our son.  Yet, if he has been ADHD the entire time we have been together, did I ever really fall in love with him?  From day one since we have been together, he has had trouble holding down a position, I was always the one taking care of the bills, the checking account and having the steady income.  Also, he has always played upon my desire to make people happy to get things, by pouting or getting angry.  He exaggerates his stories to make himself look important but usually people pick up on the fact that he is 'bs' ing them.  I think I just stayed with him because of his promises that things would get better.  I know I was naive and passive, but that is my personality. 

He says that he has changed, yet he picked a terrible time to start job searching in this economy. Plus, he hasn't done anything for a resume or applications that I've seen.  The other thing is, that I've been seeing someone else for about 2 months now.  I never, never, never, ever thought in a million years that I would be the type of person to 'cheat' or have an affair, yet, here I am.  I don't like myself for doing this, yet I feel that 10 years is enough time for him to 'change' and become the husband and father that he can be.  If he has, I'm glad for him and that will make him a better father and a future husband for someone else.  I'm just at the point that I feel, if I can have an affair, it is already to late.  I think that he suspects something, yet, I've never come right out and said that I'm seeing someone.  I just don't want to hurt him that way.  Plus, I feel that he would just say that the only reason I'm want a divorce is because of this other person.  I asked for this before I started seeing him, not that that makes it ok, I know it is wrong.  Yet, I've been telling him for years that I wasn't happy.  He really wants to try again, yet what about the other hundreds of 'second chances' I gave him?  Why should I put more time into this just to make him happy?  But again, why wouldn't I want to try if I really loved him?  I truly don't think I want to be dealing with his ADHD for the rest of our marriage.  I look at it as a disease, like being a drug addict or alcoholic, this is always a chance for relapse.  If he has to retrain his mind to think differently, couldn't it revert back?  Why should I be the one who has to remind him, redirect him and be the 'bitch'?  He has mentioned that life without me and our son is not an option, so then I feel that he is trying to guilt me into staying with him...more head games.  I really just don't know if I'm crazy or sane!!  I'm so resentful towards him, I'm just hopeless, and I don't believe his promises any more.  I know people will rip on me for cheating on him but I'll ask anyway, any words of advice?