You have had enough recent testing to believe that your heart is OK. As you have stated, the biggest problem is anxiety plus bouts of panic.
Your PCP made a nice effort in trying Zoloft on you (this is the best long-term medication I have found for my own panic attacks). But there are at least two problems here: one is that *all* the anti-panic meds have side effects, and (2) that general practitioners just do not have enough specific training and experience in the best pharmacology needed for treatment for difficult cases, which you are.
My advice: make an appointment with a real psychiatrist with a specialty in panic disorder. A good shrink will stay in close contact with your treatment, will anticipate side effects and help you handle them (most ease up with time), and will offer you other solutions for short term crises such as you are experiencing right now.
Please don't delay on this. You don't have to be a slave to cardiac fears. There really is help for you. You just have to find the right specialist in the treatment of anxiety.