All I can suggest is take it slow. Convince yourself to work for 15 minutes and then take a 5-10 minute break. I wouldn't say less than 15-20 minutes work or you won't retain any of the information. And then slowly as you feel you are able to start increasing the work time between breaks.

I found that this could often work for me when I just couldn't be bothered. I could convince myself that it wasn't that long to work and I could then go back to fun things for a bit. It was slow but the assignments did get in

It also helped to remove distractions while I was trying to work - quiet location, no phone, no other noises that could distract me from what I was doing.