I recently heard someone of the motivational speaker/counselor persuasion (mis-) using Descartes' famous quote, saying "Descartes once said 'I think, therefore I am'. If you think you are great, therefore you are great [paraphrased]".
Descartes didn't mean that. Stop turning the poor man's legacy into a self-help book.
In case you were wondering what he really meant, "cogito ergo sum" means that wondering whether one really exists is in itself proof that one exists. Go think it over.