I generally don't have this problem (I've been working mostly from home for 3 or 4 years now), but here are some things that can be helpful. You'll probably note a theme of "boundaries". This isn't all that different conceptually from setting healthy boundaries in order to maintain a work/life balance in normal office-going times, but some of the specifics can change when you're working and living in the same space.
* Have a separate computer for work and personal stuff. Don't have your work email/Slack/whatever signed in and loaded on your personal computer.
* Delete work apps/accounts (Slack, work email) from your phone. Especially now since you're home nearly all the time, you don't need them on your phone anyway. If you're signed into any web apps on your phone, sign out of them.
* Set yourself time limits. You work between X:00 and Y:00 every day. When Y:00 comes, you shut down the work computer and do something else. If this is too inflexible, set yourself a 30-60 minute range, and when you hit the beginning of that range -- set an alarm if you need it -- be mindful of what you're working on and make sure you can wind it down by the end of the time period. You can also try to schedule phone/video calls or other socially-distant activities for right around your stop time to force you to stop and get into another mindset.
* When work is done, put everything work-related away. If you tend to keep paper notes or use a whiteboard, find a place to put them so you can't see them.
* When you stop working, go outside and go for a walk. It doesn't have to be long; just 30 or even 15 minutes is fine. Don't be on your phone while you walk. Just walk around outside and take in the atmosphere. (Obviously if you are in a COVID-vulnerable group, be careful, and don't go out if you don't feel comfortable with it.)
* Wear different clothes when you're working vs. not.
* I get that you have a relatively small space overall, but if you can keep a consistent work area (that you don't generally use for other non-work things all that much), try to keep to your workspace while working, and keep out of it while not working.
* In your free time, adopt activities that occupy your mind. Whether that's reading a book, learning a language, or whatever, it's up to you. Mindless web browsing will make it easy for your brain to drift back to work, but if you're doing something mentally engaging, it's more likely work thoughts won't intrude.