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 is 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, and get used to the changes that you are facing, in order to get on with your life.
In the vast majority of cases, children get to spend time with both parents. How much time you spend with each parent, and exactly how that will work, depends on your custody and access arrangements.
There are lots of people around you who can help. Tell your parents, teacher, school counsellor, family doctor or another adult you trust.
If you aren't getting the help you think you need, keep asking until you get it.
You are not the reason for your parents splitting up. Parents split up because of problems in their relationship.
Ideally, your parents will make the decisions together about who you will live with and how that will work. Your opinion should be taken into account.