Amie D., sorry to hear you're having such trouble. I had similar issues myself and switched managers recently to a company that I'm much happier with. The old manager was very gracious and said that they were sorry to see me go, that they'd welcome me back, and frankly made the transition easy. I hope yours will behave the same way, but you need to ditch them regardless.
Find a new manager first. Do your due diligence, but try to do it quickly. Review the terms of your current agreement carefully. You may have an out for cause that you can exercise. It's unlikely that the manager will try to force you to stay - he's probably overwhelmed, but at least you want to know the particulars of the contract and what your maximum exposure is.
Once you've gotten a new contract signed with a new manager, then give noticed to your existing. Tell them to stop any attempts to lease the unit, halt all repairs, etc. You might consider giving notice earlier, but only if you're sure you can have somebody in place right away. It's not a good idea to let management lapse.
I was pretty scared about switching managers, but for me it went very smoothly and I was relieved when it was over.
Good luck!