I’m going to say two words to you right now. I want you to listen to them very very carefully, then I want you to take them out of the office with you and incorporate them into your life.
This might not work for your situation - but when I was trying to work for exams and I was getting distracted by my always-on high speed internet connection I used to give the ethernet cable to my next door neighbour to ‘look after’ that worked well for me.
I bought a timer at Target - the kind that winds up, sometimes known as a kitchen timer. I set it to 20 minutes. When the alarm goes off, it is time to STAND UP AND WALK AWAY from the computer.