With weaknesses comes particular negative thoughts. What I do is grab ahold of those thoughts and change it to a thought that I know will motivate me to better myself.
Hypothetical Example: Someone tells me that I'll never amount to anything in life, and it gets me down.
Bah, he's right, I can't make a difference in this world changes to I know I will amount to something, he's just one person. Show him up; prove to him that is IS possible, and do a better than he ever will.
The strength to overcome our weaknesses comes from within, with inner strength, magic happens.