Jules, we have spoken before and have similar HA problems. I stopped exercising as used to panic when my heartrate increased and I can hear my heart racing. Last week we were on holiday and I did a lot of walking and felt fine, but we were in a hilly area and when my husband and kids wanted to go up one area with a hill I said 'I'm not walking up that hill, I'll stay here' but then my daughter came back to tell me it was picturesque and encouraged me to go so I did and I was fine. I then managed even more steep hills when I always try to avoid them if I can because it's when my heart starts beating fast or I feel out of breath I panic. I told my daughter who also has anxiety that it's my heart beating fast I panic about and she said 'but it's only because you're exercising'! I knew she was right and I was pleased with myself I managed some big hills and felt OK.
After enjoying the walking on holiday I decided I needed to exercise more so went out yesterday and was doing OK. Normally I try to avoid inclines or hills but decided I would tackle one yesterday but while walking up it, I got pain in my inner elbow and was saying to myself 'what's that all about', then I started to hear my heart pound! It was only a small incline so I know, it was the panic about the pain in my elbow that made me anxious and increased my heart rate. I know you say you can't understand it was only a small incline but the fact your husband took a different direction and you saw an incline would have had you panicking so no wonder your heart rate increased.
It's all in the head and it's your anxiety making you experience those feelings. As said, if you have some other symptom to panic about, you forget about the current one. I do that a lot. Then, I sat on a bench for a while and a lot of joggers were running past me and I could hear their breathing which was laboured and then I thought to myself 'yes, that's what exercising does'! It's all in my head and I know it so I'm determined to keep walking like I used to years ago. Years ago I walked everywhere and felt very fit. I know that the heart rate increase is basically because I am very, very unfit!
I have never taken anything for my HA. I've never tried AD's or whatever as if I read the side effects, I would make myself experience them.
It really is all mind over matter. I think because you are not in a good place at the moment, your anxiety is hightened. Please do go on holiday and try to relax and enjoy yourself. You are letting the HA win and that's what it wants. You need to see it as your enemy and not let it win.