But most importantly, you need a job where you can do actually do this.
I've worked for a few startups (wow, ironic) where it was impossible to switch off because everything was being thrown at me as often as possible.
Yeah, they had unlimited vacation and perks like "work from home", "summer Fridays", "beer:30" but these were mostly ruses which helped them be even more effective; not me.
The easiest way to start to separate your work from your life is to move into a boring job. Sorry.
Moving fast and breaking things is cool. Changing the world is super-cool. But it's mostly passion-based work. In the heat of passion, a lot of things get overlooked.
If you work from home, first off, set aside a space that is dedicated to work. Secondly, create rituals for leaving work that include activities you never do while working. Maybe that’s reading fiction or meditating or yoga or lighting candles or singing karaoke or whatever. Your ritual should take at least 20 minutes, and not be allowed to be interrupted by work.
2) Get in the habit of writing down everything that's in your head. I keep a simple bullet journal so that everything in my head has a single place to go. Note: It's better to actually physically WRITE it than to type it.
3) Hobbies. Find something your passionate about that you look forward to doing when you're off work. That'll keep your brain off of the other stuff.
4) Exercise. A lot of time, mental anxiety can be cured with physical effort.
Those are ways I've successfully done it. Any of them work for me, but they're ordered here by the most effective (for me).
Just find a way to get it out of your head, and it should stay out until you get bored.
The hassle of maintaining two setups, having to log into the personal/work one to do something even small has helped me almost completely tune out work when I don't need it.
Tennis is my sport, and takes a lot of concentration to play well. Loss of focus is death in tennis.
Also all the focusing my eyes from near to far over and over again I think evens out the hours spent staring at a screen two feet away.
You probably don't want to sit behind a desk with a keyboard and mouse, though. Get a console.