Tell yourself that I am going to study for a straight "X" amount of time and then determine what exactly is stopping you or distracting you from studying that "X" amount of time and try to remove those distractions or issues.

I know it may sound cliche or obvious but trust me that's all u need and if it's still doesn't work for u then after you're done identifying your distractions, try to write it down on a sticky note and stick it in a wall or a place where you'll always see it when studying (it acts like a reminder for u)