Many teens really believe that their parents will get back together. They try very hard to make it happen by being on their best behaviour.
Sometimes this is a way of denying what’s really happening in order to protect yourself from the painful reality.
Chances are your parents reached this point only after trying very hard to save their relationship, and their decision to split up is final.
It's hard, but it's probably better for you to begin to accept the situation as it really is. It’s best to get used to the changes you’re facing in order to get on with your life.