Negative thoughts can drive us down. Negative thoughts beget more negative thoughts and we are in a vicious downward spiral to depression.
But hard as it seems we have a choice.
Think of your thoughts as trains and you are on the platform. Negative thoughts like blue trains (the old favourites like 'I am a failure' and 'life is not worth living') come along but you don't have to get aboard. Observe them but let them leave without you. You can get on the positive thought pink train instead!