Welcome to Mayo Clinic Connect @hopenr, I am so glad you found this online community!
Have you had a chance to read some of the HCM stories?
Yours sounds like so many others.
In my experience, HOCM was the weirdest thing ever! I had days where I felt I could run miles, and the next day I could hardly hang laundry on the line I would be so short of breath and my heart would race like crazy. I had the neck tightness too that you mention. Eating a big meal would wipe me out for the rest of the day.
I am not an expert, just a patient who was diagnosed at 62, finally, after being misdiagnosed for several years with all kinds of heart conditions...none of which I had.
I can tell you in my case, heat and high elevations did a number on me. I used to think I may be going a little nuts, because how could I have such highs and lows? I think it is all part of our HOCM/HCM condition.
I'm glad you have an echo coming up. September seems so far out when you feel so lousy, hopefully you have a list of questions you are prepared to ask your doctor. HOCM and HCM can change over time, and if you are feeling worse and worse, maybe you can ask if you can be put on a cancellation list and be seen sooner?
Has your doctor gone over the results of the cardiac MRI with you?