Hormone inbalances can simply give you all sorts of goofy thoughts. I'm a 73 year old guy, but am having pituitary issues. I also have gone through fifteen years of heart trouble, have a pacemaker, ten stents, and had a heart bypass procedure that failed, and still standing. The heart is very resilient. It would be very, very rare for you have heart issues at your age, and I'm betting very common to be going through hormone problems. My mood has lifted as I have gone through hormone replacement therapy, it happens to guys as well as ladies. Tomorrow I will go through a procedure to see if I have the C word regarding the pituitary.
I don't think you have cardiac issues, you have been tested upside down and backwards and at some place in time you should believe your doctors, and pursue hormone issues. I think with trust on your part and good blood tests to relieve any issues, you will be just fine.
I went through a similar experience after childbirth. I started having horrible panic attacks. My panic attacks were mostly related to a benign heart arrythmia that I have (which has been officially diagnosed), and which got worse after having kids. My cardiologist thought hormones may have contributed to the intensified arrythmia...I think he was right.
When my arrythmia was at its worst,, I worried about dying all of the time. In retrospect, I think hormones played a HUGE role in both the intensified arrythmia and the panic attacks. And, of course, an irregular heart beat is pretty anxiety provoking. I was in a vicious cycle! But, I am more than a year postpartum, and I am happy to report that I feel much better all around. I think you should see a cardiologist for your peace of mind, but I also think it would be useful to talk with your doctor or a counselor about your worry/anxiety, and to get that part of your life under control. Anxiety and postpartum hormones do a number on the body, especially (in my opinion) the heart. I really hope you feel better soon. I get what you're dealing with, and it's awful, but trust that your body will regulate and get back to normal!
It sounds as though you might be suffering from something called 'postpartum depression,' which you can google. It's not just 'depression,' as we usually think of the word, but a whole bunch of emotional and physical symptoms. It sometimes happens after childbirth--nobody knows why--and can sort of ruin things both for the mothers and of course for the poor kids.
It is treatable, thank goodness, so please ask your doctor to refer you to a specialist who can evaluate and treat postpartum depression.