You poor thing... :( I am SO SORRY you are going through this right now. Those thoughts are so defeating, and so scary. I am in the same boat as you and experience all the lovely emotional jazz that goes along with it. I just had my 3rd child in January and have pvc's as well.
Here's what I want you to know.....You ARE going to make it through this though. You WILL be raising them babes. You ARE going to be an awesome and LOVING Mommy and your kids ARE going to think you are the bomb dot com. These darn silly flutters are more common during pregnancy, and also common if you are feeling stressed out, but they are not harmful.
Have you tried any of the health anxiety modules that Scaredy Cat recommends? I am trying to work through them and they are pretty helpful!
Here is a link to one:
www.cci.health.wa.gov.au/resources/infopax.cfm?Info_id=53