Black and white thinking is when our brains see things as either one extreme or the other. This is a form of cognitive distortion.
Symptoms of this may be using extreme language such as: 'always' or 'never' when in reality things are usually far less static. Another symptom of this is in ones behaviour: either have the most productive day ever or not being able to get out of bed ... Working out everyday for a week and then no movement at all the next week ... Its going from 0% to 100% overnight and not allowing time for yourself to ease into new habits or commitments.
This is very unhelpful as it is impossible to maintain most habits when they are done to the extreme.
What can help is asking yourself what could lie in between - can you find a middle ground between 0% and 100% and intentionally choose somewhere in between.
If you catch yourself using extreme language - stopping and rephrasing it; whether it's in your thoughts or out loud. As when we use these words they trick us into thinking the extremes are the reality and get us caught up in our emotional minds rather than our rational minds.
So think about what the facts are and reflect on the reality of a situation and rephrase the extreme language you find yourself using.