Roommate struggles

My roommate is hoarding our heating bill. We have a "pay at will" - an option to pay more in the fall and pay less in the spring. Well we paid less in the fall, so I know that the bill will be higher now. 
We have our lease until October but I am planning to leave my room in August. I want to pay all my heating before August begins so I am even. 
The problem begins because she is literally insane. She sabotages me a lot - waking up when I have the bathroom and going in two minutes before my time, steals my stuff, being super loud, bolt locking me out. I have talked to her about but she just says sorry and continues her behaviors.
I have spoken to the landlord about leaving early. I don't want to tell this crazy roommate that I'll leave because I'm sure she'll do something to ruin it. Any thoughts on how to get the login info to pay the bills or how to get her to tell me what I owe without being suspicious?