It sounds like you made the right decision, even though I'm sure it was a hard one to make. I think life changing decisions always end up with some part of yourself doubting what you've chosen.
Fibromyalgia is awful (my mom has it) and if you've found a job that doesn't cause you pain....then go for it! Pet sitting sounds like it makes you happy, which should be your highest concern.
I'm sure your hair clients will understand your decision once they have time to think about it and get settled in with a new stylist. You can't live your life to make other people happy or keep from hurting their feelings, etc. If they are truly as close as you say, then they'll understand and support your choice.
Good luck and enjoy your free time. After 18 years I think you deserve a major vacation!! Maybe starting to plan a fun trip will help you forget about the guilt for a while.