Every time you are about to put yourself down, please think of this image. Days, months, years tend to pass extremely quickly, and when you get to an older age, the worst thing will be to regret not letting yourself be truly happy.
You must, above all else, be on your own side and build yourself up, rather than knocking yourself down.
So next time you are about to say something negative to yourself, stop. Take a breath, take a walk, think what you would say to a friend in the same position. I bet you would say something quite different.