No matter who you are or how successful you are, everyone has periods of "depressing" on life or on yourself.
These periods of "depression" lead to negative thoughts, which can lead to low productivity and, for some, to different levels of depression.

Here are some inspirational messages you can use if you wish:

Christopher Morley

There is only one success--to be able to spend your life in your own way.

Earl of Beaconsfield

The secret of success in life is for a man to be ready for his opportunity when it comes.

Napolean Hill

The majority of men meet with failure because of their lack of persistence in creating new plans to take the place of those which fail.

Josh Billings

Consider the postage stamp; its usefulness consists in the ability to stick to one thing until it gets there.