I struggle with it a lot and only recently have I figured out a system that works for me. My problems are I can focus forever and if I'm not working, I feel like I COULD be working.
For a number of years, I'd commute 2 hours to work, work, commute 2 hours home, shower, eat dinner, then work on personal projects or my portfolio until about 3am. Fat lot of good it did me. Got hand damage and not very far lmao so I learned my lesson.
I figured out that if I allow myself the day, I'll spend the entirety of it working. If I put hard hours on myself and say that rest is work, I can trick myself that it counts. So I work from right after breakfast to lunch (I also use this time to shower or workout), allow myself an hour lunch, then work until dinner.
Since my time between breakfast and lunch is shorter, I spend that time doing menial things, like emails, since art or drawtectives takes more brain power to boot into and stay with.
I don't have any issues focusing, so this method works for me. I just put on a show or movie I've seen a bunch or a podcast and just go.
I wish I had some more time to do some other things, but if I do personal art at all it'll get me too close to work, so I have to do that sparingly.
If you want to see my full breakdown, I did a drawclass that includes time management. You can find it here!