How many times have you told yourself something negative about yourself? What if you took a percentage of those and changed them around. Pick one. Just pick one thing you tend to say.
For example: I forgot to do this!
Turn that one statement around and say: I am remembering to do this.
Because if you are remembering that you forgot you are beginning to remember. And if you practice that thought a bit more - when you remember things instead of forgetting them - like running back into the house for your keys: "I remembered them!" you will build a better self-talk system.
But don't try to do it all at once. Just pick one thing you tend to say to yourself and work on lightening that one up a little bit.
Each time you find a new theme of thoughts in your mind about yourself to lighten up the whole paradigm begins to shift.
Until you see yourself as we see you: a perfect consciousness going through imperfect-to-you situations, refining them.