You were wise to refuse to become the mediator in your parents’ problems. Even at 25, you are still their child. You love and are loyal to them both. It was unfair of your mother to try to put you in the middle. It’s unfair for both of them to be complaining to you about the other. It would be detrimental to your relationship with each of your parents if you were to try to step in.
Your parents’ relationship is between the two of them. You can’t fix it. All you can do is tell them that you do love them both, that you miss the way things were, and that you wish they would stop fighting and would get the counseling they obviously need. If their problems could be solved by fighting, they would have figured it out by now. Tell them, as calmly and lovingly that you can, that if they start to complain to you about each other, you will simply leave the room. It’s possible that if they don’t have someone to complain to, they will have to deal with each other directly.
I’m very sorry for your loss of the family you remember as being happy. It is very, very sad for everyone. But you really do need to let them work it out on their own. At your age, you have plenty of other things to keep you busy. The task of the early 20s is to find a career that satisfies you, good friends to hang out with, and someone to love.
I wish you well.