Hey Andrea!
I also am a heart worrier. One thing I've found that helps is to make yourself not check your pulse. I've had a hard time doing this myself, but when I have days that I keep noticing it and keep checking it, I remind myself that your heart rate changes constantly throughout the day for many, many reasons - your hydrations levels, your activity level at the moment, your emotional state, how you're breathing, how much sugar or caffeine you've had throughout the day, the temperature, the way you're sitting, even standing changes your heart rate. MANY things will change your heart rate. I read that the only accurate way to measure your heart rate is right after you wake up in the morning. Not when you're lying down because lying down makes your heart beat faster, but after you've gotten out of bed and are sitting comfortably. Before you've had anything to eat or drink because this will also change your heart rate. Anyways.... check your heart rate in this relaxed state. Anything between 60 and 100 is normal. But remember that it will change drastically throughout the day so this is really the only time you can get an accurate reading. When I'm noticing my heart racing throughout the day and I can't seem to stop thinking about it, I tell myself, check it in the morning and you will see that your heart rate is ok. And it always is. I still have days when I struggle with remembering this, but it is helping me to calm down about it for the most part. I hope that helps you also.
About the pains in your left arm - I have this problem as well. My left arm kept going numb and I kept getting sharp pains all the way down to my fingers, even when I was relaxing. Naturally, I thought I was having a heart attack and went to the hospital. But, what I found out is that it was my back - not my heart. They told me to stretch my arms and my upper back muscles every time I get this pain and that it will take care of it. And it does! Roll your shoulders back a few times and foward a few times. Scrunch your shoulders up to your ears really hard and relax them a few times. Put your chin to your chest and you will feel all the muscles in your back that are hurting - roll your head a few times back and forth because it really stretches out those