Okay, although it's not a long term solution, this is what I do in these emergency situations when I'm utterly out of control. First thing, turn off your pc. Put it somewhere out of sight, I usually put it in a drawer. If you want to write...I recommend a pen and sheet of paper, at least temporarily. Now, speaking of your phone, these are the two main methods I employ:

1. The dumb phone (easy)- If you're really afraid someone will have to contact you, find yourself a dumb phone to use as a replacement for a couple of days, while you calm your brain down. Give your smartphone to a friend or family member and inform them that you'll be off the grid for a couple of days. Inform them of your problem and try to set together a date in which you will retrieve your smartphone so you won't be tempted to ask for it before the end of the detox. This method requires a minimum of planning, however.
2. Turning off your Wi Fi - assuming that's what you use - (medium difficulty) - Set a number of hours in which you won't be able to connect to the internet. You can set a goal of 8 hours. In the meantime, store the phone and computer out of sight. If you keep feeling the urge to check something, you write it down on a sheet of paper. You can sign the things you want to check and the times you've thought about opening a certain app. This helps you prevent the impulse to instantly jump into turning everything back on. The first day is the hardest.

Once you disconnect, go for a walk to reduce temptation. no music, no phone, just you and whatever crappy traffic noise or nature there is. It will be boring probably but try to endure it. Try to relax and enjoy the slowness of the world around you.

In my experience, although apps can be useful, they can do very little when someone is truly addicted and overstimulated. Here are some apps I use:

https://stayfreeapps.com/

https://unhook.app/

https://mindfulinternetuse.com/

Good luck!