Your ability to get lost in a task or client project does not justify the extra time. Budget a block of time for each task and then get it done in that amount of time. If it takes you longer than you expected, make a note of that for the future. In this project, you'll eat the extra time expense, but then you need to do one of two things going forward: get faster, or adjust your expectations.
The tighter you control your time task by task, the tighter you control your income potential.