There is of course a rather large difference between "Risking burnout" and "Being a lazy slacker".

Have you done a reasonable amount of work on a project in the last while? - Well then don't worry too much about taking a break, exploring other things, and remembering to enjoy life. Read, Watch movies, play games, go for walks, and get away from 'work' things on a regular basis.

Keep a calendar of what you've done. For hobby stuff I like to set shade in days with red/green markers. Did I work on the project for the 1-2 hours a day? It gets a green square on that day. Did I slack off and not get anything done? Red Square. Did I achieve the weekly minimum of 6 hours? Then i get a green bar. Otherwise it goes red.

After awhile trends and patterns start to become visible. Seeing lots of the red is a good reminder that I've been slacking off, and should avoid the temptation to take a "relaxing evening" because "I deserve it...", because why do I really deserve the relaxing evening if I haven't been making headway toward the goal I set?

That said, don't bash your head against walls of problems you aren't making headway with. Look at things carefully. Take sanity breaks as needed, just remember to track how much you are and are not working to avoid letting things slide too far one way or the other.

STOP.

If you want to do something more than you are already, let it be something not mind-based.

I suggest jogging. Or fast-walking. Go have an ice cream to the other side of town. Whatever makes it for you.

I currently work 0800-1730 but with a 80 minute commute that becomes quite longer. This is hopefully changing now and I plan to go on swimming.

I don't think you need motivation at this point. You need to manage yourself for long-term effort.