SAME thing has been happening to me lately! Went to ER and a follow-up w/ a general practitioner and both said that it's super normal to get an "early heartbeat" occasionally and/or palpitations. I've also had chest pressure, which adds to me thinking I must be having a heart attack or whatever. It doesn't help that I worked @ a cardiac & vascular clinic/hospital for 5 years - I have all kinds of knowledge about symptoms of any kind of heart problem. And they're the exact same as anxiety symptoms!
Perhaps you should look in to seeing a different doctor, one that understands anxiety disorder and is maybe willing to prescribe medication?
I'm currently taking Gabapentin for my anxiety. It's commonly prescribed for seizures (which I do not have) and nerve pain (also, don't have that). My MD wanted to try it because it's not addictive, and I have a history of addiction in my family, and I really don't want to do benzos because of that. However. The Gabapentin is not really working anymore (been taking it about 8 months), and my anxiety is SO BAD that I can't even relax on the couch & like, cuddle w/ my BF (he's so sweet, bless his heart, he gives me a massage - he's a certified massage therapist - every night!). I can't even relax enough to lay down @ night to go to bed - I have to like, prop myself half-sitting up, against the wall. I've had nights where I've literally been on my knees crying like, WTF I can't take this anymore!!!
Wow, way to ramble, me! Anyways, it's good to know there's other people out there that understand what it's like to go through this awful condition.