I'm in the same boat as you. Even if I have free time in the evenings or weekends a lot of my desire to be in front of a computer is gone. I've been slowly chipping away at a side project but I feel like if I wasn't weighted down by my day job I'd be able to get on with things faster.
You could find a smaller problem to solve. Get up an hour early and work for yourself.
Quit.
Maybe good advice, but I'd recommend having a few months wages as a cushion.
Better to have an MVP and a few customers (paid or expressed paid interest) first. That's what I did + the cushion.