Write your Sad times in Sand,
Write your Good times in Stone.
- George Bernard Shaw

Instead of dwelling on negative thoughts,
cause your mind to dwell on peace and joy.
- Ernest Holmes

If only the people who worry about their liabilities
would think about the riches they do possess,
they would stop worrying.
- Dale Carnegie

He who spends time regretting the past,
loses the present and risks the future.
- Quevedo

Further reading: 12 Life Lessons I Learned From My Cat

Feeling Depressed?

When It's Darkest

Count Your Blessings - Not Your Troubles

Overcoming Anxiety

Five Questions to Ask Yourself When You Feel Sad for No Apparent Reason