I spent over 23 years as a hair designer. about 10 years in I had to cut my hours to part-time. The pain was so intense and nothing helped. A few years into the part-time reduction I figured that I couldn't even do that anymore. I needed a career change and spent a year trying to figure out what to do. I have always been a big animal lover and voluntered with a Boxer rescue. This lead me to helping out hair clients through the years and pet sitting for them when they traveled. That part-time side business lead to referrals to people that I didn't know and I started getting nervous entering peoples homes that I had only met once. It was fine to visit my hair clients homes since I had known them seemingly forever!
Another year of researching professional pet sitting and what all it entailed lead me to starting my own legitimate company. I still had hair clients to care for so I bought into a friends salon so that I no longer needed to pay rent weekly (I had been self-employed as a hair designer). I started advertising pet sitting, got certified in pet first aid and CPR, created a website, bought insurance etc....
I booked pet sitting jobs around my hair schedule. In under 6 months I had to make yet another change and started booking hair appts around the sitting schedule. about 3 years ago I finally dropped hair down to 3 days a week, about 10 hours total. Last month I dropped Saturdays doing hair (YEAH) and now only get to the shop 5 to 8 times a MONTH, never more than four hours at a stretch. That is all that my body can handle of that profession. I am trying to find help for the pet sitting.....business is booming! I have fought making a decision to stop doing hair altogether but I have some really great clients that won't let me and then add in family and friends and it's just not something that I feel I can let go of entirely.
I would have been the LAST person on earth that would even think about changing careers. I was a lifer! Maturity set in and with that came the fact that I wasn't physically capable of doing it day in and day out any longer. I felt like a failure but kept on trudging along. I can only thank God for clearing my vision enough to see that I was fully capable of doing something else and the work with animals was my TRUE calling. I KNEW that but didn't know what to do about it for years. I thought volunteering was the answer. While I love that part of my life it doesn't pay the bills.
My job now is hard I won't lie. I am open to business 7 days a week, 365 days a year. I can work for months without a day off but this work doesn't HURT like doing hair does. When I do hair now I know that my limit is 4 hours at a time. If I try and go longer than that I pay for it for days. Getting up early for pets is the most difficult. I move SLOWLY in the morning and it ain't a pretty sight! But the critters don't seem to mind. 
I'm living proof that if you can physically do "something" and want it bad enough it can be done. You just have to find that one thing that will work for you physically, mentally and spiritually.....it's also just an added bonus if you get paid to do it!