Pro tip when you easily master the 25 minutes: don’t let a pomodoro timer interrupt your flow. Just start a time tracker and when you feel that your mind starts to wander, take a break. I often reach 55 minutes like this.
Also, don’t take one thing at a time too literal. When you are working at multiple connected things, call these connected things one thing. Like a painter who works on multiple spots of the painting at once, this is sometimes very necessary as long as you don’t feel overwhelmed.