It is easy to say negative things about yourself when you are feeling down. 

I am not good enough. 

I am lousy.
 

I can't do anything. 

But we are not. One mistake does not make us a bad person for life. And it's not like we are facing our life alone. God is with us. He never leaves us nor forsakes us. 

I may fail this time but Christ will use this failure to open another door of opportunity.

I can do it through Christ who strengthen me.

Nothing is impossible, when Christ is with us. 

Nothing formed against me shall stand.

We may be lousy if we don't have Christ in us but with Christ, we are champions. And a champion, does not say bad stuff about him/ herself.