Sorry to hear you have this symptom. I used to get it, too, and it is one of the worst symptoms to try and tolerate. It feels terrible and it's scary. I think mine is caused by Babesia, but who knows. Before I knew I had lyme and coinfections, I went to "regular" docs and they put me on a beta blocker to regulate my heart rate and blood pressure (I had high blood pressure too). Thank goodness the drug has really helped me. I also think I have low thyroid and do not know how that may be contributing to the heart rate symptoms. I will have to ask the doc again. When I was on meds for babesia, the symptoms were worse even with the beta blocker, so it makes me think my problem is due to the Babesia. I hope your doc is able to help you find some relief. Probably good detox methods would help?
A good friend of mine with lyme/coinfections also gets the heart symptoms from sensitivity/allergic reactions to food or chemicals/smells etc. Maybe you have acquired some allergies you did not have before? Might be worth finding out. See if you can connect the negative heart reaction to something you eat or touch/smell etc.
Best wishes to you.