A plan can sound sensible and still fail at the point of action. You decide to spend the first 30 minutes of the day answering email before opening social media, then pick up your phone five minutes after sitting down. By lunch, the important task remains untouched.
The problem may not be weak discipline. Often, the surrounding conditions make the unwanted behaviour easier, more familiar, or more immediately rewarding.
Effective behaviour change examines the setting around an action and adjusts the cue, the action itself, or its immediate consequence. The aim is to make a useful behaviour easier to start and repeat, while accepting that every change uses some time, attention, or comfort.