When I was a teenager I did that once. I learned quickly to stay out of their marriage.
The issues your parents have are not because of you nor are they your problem to fix.
I used to feel that because they were my parents and their relationship affected me and that I had the right to say something.
I learned I don't and that when I did try I was only hurting myself in the process.
Unfortunately when your parents are not happy it hurts you and you just want it to end, but because you are not the reason of their issues you cannot fix them.
You have to let it run it's course and try to find a way to shield yourself from it and to not get involved. It's hard, but possible.

Hang in there...