How do you keep focused on what you really planned to do, rather than always getting new ideas and following them rather than the actual task?

I make prioritized to do lists at the beginning of my work day. Things that are prioritized and #1 have to be completed first be before move on to other ideas.

I struggle with this exact issue and what I found works for me was adding the One Big thing habit to my routine and setting aside a decent amount of time 30-60min and training using that time to focus solely on that task with no distractions, sometimes having a time limit and knowing theres an end in sight help me to stay focus where as if I try and focus on one thing for 'as long as it takes' I find my mind and thoughts and focus wander for sure!