I'm so sorry for your situation and can understand the mixed emotions during this time. It is difficult to understand why they have stayed together but I think you have to accept it was their choice. They cheated you but also cheated themselves out of alot of happiness.
You need to prepare yourself emotionally for your visit. It is quite common for chemo patients to lose hair but I was unaware of the glaucoma side effect. It is hard to watch anyone suffer. You are a very caring daughter, and I believe, capable of forgiveness too. I think you may need to do that at some point in order to let go of that anger--not that it isn't justified, but for you. You deserve happiness in your life.
I can only tell you what I would do. I would visit but would cut my visit short if the atmosphere becomes uncomfortable. Maybe you could even go to see her when your Dad isn't there?(that way they can't argue) Also, I think I'd continue seeing the therapist if it helps. I wish you the best.
Dwelling on what could have been or should have been is most counterproductive. And, the negative (angry) feelings will only fester within you and weigh you down. Rejoice in what ever goodness your mom had; forgive your dad; learn from your parent's mistakes; and vow to be a better, kinder and happier person and parent than they were.
I'm so very sorry for all the pain you have suffered and the illness that your mother is facing. My situation is that my only brother married and my mother died a week later, the following week her brother in law died, the following week, my exfiance's father, whom I was very close died (another funeral), two months later, the following 3 weeks, 3 family friends died of cancer (more funerals), 2 months later, my mother's best friend who attended moms funeral died of cancer, my brother's wife loss there first pregnancy two weeks before Christmas, loss my job, and so on. I became so physically sick, that I couldn't cry, because my throat would swell very dangerously to shutting. It is a very difficult journey, but God gave me strenght to "survive".
All you can do is just be there for your mom. Let her know she is loved. The past is the past and you will not be able to function properly to what is to come with the past in the way. As for your parents, it's very important to forgive. You must forgive them both at this time no matter how much the squabble with each other. There will come a time when your father is going to look back at his life and realize his actions. To forgive is "divine"...not easy, but it will make a better person out of you.
Right now, I'm still healing and I can only describe it as a nightmarish journey that you wish upon no one, but you will have inner peace knowing that you did everything possible for your parents regardless of how abusive there were in the past.
We are here for you and it's going to be ok...one day at a time. Judy
p.s.....you can still have a very good family life. Brake the cycle and start a good family life with you. Judy