Oh man, I'm in the exact same spot right now. I'm 35, sold first website when I was 17. And I don't even have "regular" job now, just working on my own projects and I still completely dread starting to work every single day. Sometimes it takes around 6 hours for me to just get started and write a line of code. Only to stop an hour later because I'm exhausted from doing nothing.
The only time I am able to regain that "in the zone" feeling from 15 years ago is when I try some completely new stack/framework/language on a fresh project. The problem is I can't do that all the time because I've started dozens of projects and wasted years without finishing anything.
I will be monitoring this thread for any help. This is a serious problem for me.