I hope you are right with what you want to do. This manipulative pyscho will most probably cause some type of scene before she goes. She's loosing what she views as her property, that would be you.
These types of unstable people were taught by their parents to be abusive and they will resort to the same tactics they watch the parents use to control them and the other person (the husband or the wife). They rarely "just go away". You are right to let her think you'll be friends after she leaves. That will let her think she still will jave a chance to control you later.
But my advise is to break all communications as soon as she's out. Let your parents know that she will try to contact them and say hurtful things to get back at you, Be preemptive in dealing with whatever parts of your life she can mess with. change your phone number, change banks, change any habits that she can predict (i.e. going to the gym or other hobbies, etc.).
Whatever she knows about you she will try to use to mess with you and cause you problems.
I had a gf once who was so obsessed, she went to my work and told nasty lies to my workmates that included me being homosexual and using her as a "cover" to keep my high profile job. Don't get me wrong here, I have absolutely nothing against gays and lesbians. I am on occasion bi with the right man, but I work in a profession that is full of close minded old school bigots, nut they pay me extremely well for what I do. So her trying to "out" me and mess up my income was just a tool in her arsenal to cause me pain and issues.
If she couldn't have me, she was going to make sure no one else wanted me. She even followed a girl a met 6 months after we split to her house and threw a wild tirad about how she and i were still together and told this girl to stay away from her husband. We were never married, never engaged, never lived together, and only dated for 6 months.
This went on for over a year after we split up and it became a job, just running around repair the damage she caused me. It only stopped when I got a transfer to another state.
I hope you don't have to deal with this extreme type of crazy, but i recommend being preemptive. Change or insulate everything about or in your life that she knows how to mess with.
But most importantly, cut off all communication as soon as possible. Be sure to change all passwords to all your emails, bank accounts, etc. She will try to snoop in order to get info to use against you and just to cause you problems.
Best of luck to you and may all your dreams come true.
Anything worth having will take a great deal of effort!