Why do I still want her and not just let go? What is wrong with me?

I am sorry to hear you are going through this. It is understandable that you are in pain. You still want her because this is sudden! You have spent nearly 20 years with someone and are now facing a big change. Many parts of your emotional and financial security are being impacted. You have also learned about an affair in the midst of being rejected. Do not have unrealistic expectations of yourself. You are a human.  

As we age, we go through various life stages. What we want out of life can change from what we wanted even a week or a year ago. She may have experienced a change in her outlook. This is not indicative of your quality as a person. People's behavior has more to do with them and their own experiences rather than those whom they are interacting. 

It would appear that she has already processed many of the emotions you are experiencing and is in a different stage of change than you. There is nothing wrong with you. It is normal to feel saddened and even abandoned or betrayed. The question now is one you have already asked yourself. You are fighting to keep the relationship that you were content with. However, it is not healthy to convince someone else of your worth and value in a relationship.

I would suggest placing boundaries regarding the conflict and utilizing radical acceptance.  This does not mean that you have to approve of the situation. You must accept it for what it is and believe her feelings are valid to her, even if they do not yet make sense to you. Abuse is never acceptable and does not solve anything. Please make sure you are setting boundaries in these discussions and enforcing them when you find that they are becoming unproductive and combative.

Do you have other social relationships that can be utilized for support? This is not something you should be going through alone. The truth is that you may never understand why she feels the way she does. The important thing to do right now if focus on what your needs are and reach out for support. I know it is painful right now, but I assure you that these feelings will not always be this heavy.