Hello, I am very sorry for what you are going through. Actually, sometimes something similar happens to me at night, I start to feel my heart beating as if it were going to go out, I start to despair because I want to calm down, and I can't, and I start to feel afraid that I might have an attack or something like that.
It has already happened to me several times, and it is almost always at night, when I go to sleep. Lately, when I have had these heartbeats, I told myself that it will not overpower me, so I start taking deep breaths and I start to let the air out little by little, until I can control my breathing. I start singing a song in my mind or out loud and believe me it's hard to remember a verse, but I try over and over again until I get it right. Or sometimes I get up and start watching something funny on the PC or TV, something to distract my mind and not listen to my heartbeat. These techniques have worked for me and I know that I have to go to a doctor to get checked or tell me why I have these sudden attacks, but the truth is that I don't like doctors and just thinking about it makes me more stressed, but at some point I will have to go.
Yeah, sounds a bit like you have white coat syndrome! My wife has that too. She doesn't like going to the doctor at all. I try to not move around as much as possible because I don't want to wake her up. I feel like I should maybe take my blood pressure when one is happening, but I haven't done that yet. It's a horrible feeling!