I need my parents to be together

I think the best way is to ask them. You are not a child (assuming you really are 18 as your Yedda profile says), you are mature enough to know the facts even if you wish them to be different, and you deserve to get all the information.

Before you ask though, make sure you are ready to accept all answers, not just the ones you are looking for, because then your parents will lie to you to keep you happy and ignorant. 

(Using my voice of empathy) Ask them.

Ask your mom, "What did dad do to make you want to leave him after I graduate high-school?"   It seems unfair that she will not divorce him until you graduate which is 2008, 2009 or 20??. Wow, she really did give you a graduation gift early.   Ask mom, what is the problem between her and dad.  Then you can determine if they need to stay together or leave each other.  If dad or mommy cheated with other people, do you think it would make them feel better to live together and make each other angry while they truely may want to be with someone else.  Ask mom, why wait and why not now, because you'd like to know.