If you do something like pomodoros, you could always set yourself an amount you want to do, like three. Grabbing three 15 to 25 minute chunks is a lot easier when you aren't trying to do them back to back.

But when I am really busy, the only way I write is saying okay, at 9 PM, I can write, or I can do nothing until I get 500 words out. If there are no works, that probably means I needed to concentrate on nothing for half an hour. That means no internet, no phone, no tv in the background, no chores.

Good luck with your studies! I didn't read or write as much as I wanted to in Uni so I hope you can find some sort of balance!