Then do what the OP is contemplating doing if you can (fair enough, I don't know your circumstances) and have a change if possible. Study for something of an evening and a few hours at weekends whilst working in your current job? I don't even know if you do earn £15k pa, or you were just...
I've earned £100k+ a year and been thoroughly miserable. I've been sat in a private cabin on a First Class flight to New York sipping Champagne, thoroughly miserable... whilst being surrounded by thoroughly miserable people. I've had dinner at the Burj Al Arab, surrounded by miserable people...
I've been through similar to you fella. I was self-employed in the Motor Trade for 14 years. Loved it to start with. Huge laughs, great money, jolly boys club, fantastic memories - couldn't wait to get up in the mornings. It then changed beyond all recognition and for the last 1-2 years I hated...