I also had palpitations while waiting for surgery to repair a severely regurgitating mitral valve. Most severe at night and especially on the left side. Somewhere I read an explanation for that but I don't remeber what it was (check the webchats about irregular heart rhythms on Cleveland Clinic). For me it seems to be triggered by anxiety- the palpitations got worse after I found out I needed surgery, and settled down once the date was set and I had something to plan for. They started back up before my one year checkup, nerves again I am sure. I know this sounds ridiculous, but try to calm down. If you are one of those people who is comforted by knowledge, there is a lot of excellent info at Mt. Sinai's website, mitralvalverepair.com or something like that. They are pretty close by you and a good place to go; lots of patients on this site have had surgery there. Just know you are not alone, and many have successfully gone through this before you! Best of luck as you start this journey,
I had really bad palpitations before my surgery also, and like you, it was very difficult to sleep hearing and feeling all the pounding.
The only relief I could get was to sleep on my right side. It didn't stop the pounding, but it was better or less noticeable on my right side than when sleeping on my back or left side. Sleeping on my left side was almost impossible.....felt like a fish flopping around in my chest.
Good luck...hope it helps.