I woke up this morning thinking about sunk costs or rather how sunk costs can get in the way of progress. Have you ever taken that first step in a effort to change only to have your head fill up with all the reasons it's not going to work? Among those reasons is the fact that you've done it that other way for so long and you made quite an investment (money, time, energy) in doing it that other way that there can't be any chance that this new way will work, so you should just quit now. In accounting, we called all of those investments made in the past sunk costs. I see past habits as part of those sunk costs.
If you have set your sights on a new destination, you can't let sunk cost play a role in figuring out how to get there. You have to throw out all of those habits and attachments to your past investments. They're irrelevant to your new path regardless of how important they were in your old. They'll only slow you down and may even stop you from getting to that new place.
So, the minute you find your thoughts wandering in the Land of the Past, stop it. Just focus on mapping your future.