You poor thing, this sounds really tricky to cope with. I have a similar thing that runs in my family- skipped heartbeats (where it feels like your heart has stopped dead in your chest) and then lots of quick little beats. To me, it feels like a bird flapping its wings in my chest. All the women in my family have it, and it still shocks me a little bit every time I feel it!
I've had mine checked out a couple of times, and although it picked up some ectopic heartbeats I was told it was nothing to worry about. I'm not generally anxious about my health so I just try to remind myself that it feels awful but I am safe.
However, I know that there are two things that make these weird heartbeats VERY frequent- hormones and stress. My period or being pregnant makes them occur many times in a day. I don't have a MH problem but know from experience that any emotional stress makes them much worse. Interestingly, the other symptoms you describe- almost like a lump/obstruction in the throat and chest tightness were symptoms that I experienced alongside the funny heartbeats during a period when 3 of my grandparents died one after the other as a teenager. As the stress of that grief lifted, my symptoms gradually left, although it took some time.
I really wouldn't underestimate the power of stress to make this problem feel really quite severe. The more stressed you get about it, the more it happens and it becomes a vicious circle. I'm probably only relaxed about it because my grandmother and great grandmother had exactly the same weird heartbeats and both lived into their 100s, fit as a fiddle.
I really would try the medication your doctor has suggested. Also, try to find some strategies to calm yourself when you feel these strange heartbeats. It sounds like you are very frightened so I don't see anything wrong with seeking a second opinion to reassure you, but in the meantime try to find ways to make life a bit less stressful. I found meditation, yoga and plenty of distraction are useful tools. Best of luck xx