If you chase two rabbits, you will catch neither one.

Russian proverb

To become a master single-tasker, adopt the mantra "Do-One-Thing (at a time) and Celebrate."

Create a code or brain script for yourself, such as "DOT+C" to help you memorize it.

Let me suggest writing this out 10 times a day for 10 days:

1. Do One Thing and Celebrate. 
2. Do One Thing and Celebrate. 
3. Do One Thing and Celebrate. 
And so forth...

Use BJ Fogg's Tiny Habits methodology for coming up with a celebration after completing each singly done task.

BJ Fogg's Tiny Habits methodology is a behavior change framework that emphasizes the importance of starting small and focusing on building new habits incrementally. It suggests that small, easily achievable behaviors, repeated consistently over time, can lead to significant and lasting changes. According to this methodology, to create a new habit, one must identify a trigger, perform a behavior, and celebrate the behavior immediately after performing it. The goal is to make the new behavior a natural part of one's routine by repeatedly linking it with a well-established trigger. This approach is designed to be simple, scalable, and sustainable, helping people achieve lasting behavior change.

I don't know if this is for you, but to celebrate you might want alternate between a deep squat and a push-up after completing each singly done task.

For me a simple exercise movement will be a perfect celebration to cement the habit.

It's easy. Focus on one task at a time and celebrate each accomplishment to reinforce the habit of single-tasking.

I guess the next question to ask yourself is, "which rabbit?"