Twenty-two affirmations to change your mind about yourself — for when the harshest opinion you hold is the one about you.

You will speak to yourself more than anyone else will speak to you, and for most people the tone is one they would never use on a friend. That gap is not humility. It is a habit, and habits can be changed.
These twenty-two affirmations are aimed at the habit rather than at your self-esteem. They are about what you let into your mind and what you say back, which is the part you can actually operate.
Trying to jump from "I am useless" to "I am brilliant" fails, because the second statement gets rejected on arrival. The workable move is tone first: same content, said the way you would say it to someone you liked.
Then interrupt rather than argue. When the harsh line shows up, naming it — "that is the old voice" — does more than constructing a rebuttal, because rebuttals keep you in the conversation.

1. “I know that my happiness depends on me and my mindset, not on external circumstances.”

2. “I am capable of anything I set my mind to.”

3. “I have control over my mind and filter the thoughts that bring me down.”

4. “My mind is a powerful tool, and I use it to create my future.”

5. “I focus my mind on my goals and ignore unnecessary distractions.”

6. “I am happy because I have made up my mind to be.”

7. “I feed my mind with positive thoughts and overwhelming love.”

8. “I choose to fill my mind with positivity and strength.”

9. “I believe in my ability to achieve whatever I set my mind to.”

10. “I notice how utterly important it is to keep my mind positive.”

11. “I feed my spirit. I train my body. I focus my mind. This is my time.”

12. “I am ridding my mind of negative thoughts and filling it with positive ones.”

13. “I am capable of everything I set my mind to achieving.”

14. “I know I am good enough to accomplish whatever I set my mind to.”

15. “My mind has unlimited power.”

16. “My mind is a magnet for positive experiences.”

17. “I am patient in educating my mind to be more positive.”

18. “My mind creates only positive thoughts that help my body to be healthy.”

19. “I am kind to my mind today.”

20. “I surrender my mind to positive thoughts.”

21. “I only let positive thoughts into my mind.”

22. “I am in control of my mind and my thoughts, and I choose positivity.”
Yes, though more slowly than most advice implies. Self-concept updates on evidence and repetition, which is why behaviour change and self-talk together work considerably better than either alone.
Frequently because it is — a parent, a teacher, an early boss. Recognising whose voice it is makes it much easier to treat as an opinion rather than as a fact about you.
Then move to neutral first. "I am learning this" and "I am allowed to find this hard" get accepted where praise does not, and they still interrupt the harsh line.
Weeks for the tone, months for the belief. The first sign it is working is usually that you catch the harsh thought rather than that you stop having it.