Some work is taxing. Often it is work that if someone else assigned to you, you would do gladly because of their expectation of you to finish it. However, when you are only relying on your internal expectation to do the work, I notice that you get off-task and don’t stay as focused as you should on the task at hand. There’s no excuse for this.
The mentally draining work is what you have to do considering none of the work you do has been against your will. You decide to take on all of the projects that you do. By not focusing you are only delaying the inevitable, which is that you have to complete these tasks.
I’ll prescribe you a bit of single-mindedness and solitary confinement: put yourself in a room with nothing else to do but the work in front of you. What else matters at this point? What else should you spend your time on?