When I was little I told my parents I wanted to be a "chireman." I couldn't say "fireman" at the time, but that wasn't going to stop me from saving people from burning buildings.
Then it was Batman. Then it was a full-time musician. I'm none of those things now (though I'm still working on the Batman part), and that's totally okay.
It's okay that you aren't doing what you dreamed you would be doing.
You don't need to go back to school. You don't need to quit your day job. You don't need to despair. You're not a failure.
I rarely meet a person who decided on a path they never veered from. Most people pick a path, deal with some trial and error, make adjustments, and find what's right for them.
The true joy and satisfaction that comes from doing something you love can still be experienced outside of a full-time, professional career. You don't have to get a paycheck for it to be worth your time or to have an impact:
I have a coworker who loves comedy and standup and makes it a point to do a bit at our company meetings a couple times a year.
I have a friend who is great at drawing, loves the idea of illustrating, but sells real estate for a living.
Tons of my friends are great musicians, work full-time jobs, and play music on nights and weekends.
I have a mentor and friend who has pastored a church for 18+ years and works another job during the week.
The craft is more important than the career.
Just start. Pick up the guitar again. Grab that book you put down. Start drawing. Write. Build something. Create a side business. Whatever it is...you don't have to abandon it because it didn't turn out exactly how you expected.
Maybe it will make sense one day to start over, pursue that path one more time, or quit your day job. In the meantime you don't have to give up the joy, though. Love your craft. Don't worry about the career.