A voice in my head says I can’t write every day?
I’ll write 2 sentences every day, just to prove to that voice that I, in fact, CAN write every day.
A voice in my head says I don’t have the perseverance to train for (and then finish) a marathon?
I’ll do something small to prepare for the marathon every day, so at the end of each day, I can say to myself “The proof is there, today was another day of me persevering and preparing for a marathon.”
You can’t brute-force your way out of an “I can’t do this” belief. You can only take small actions that start proving the contrary.
Slowly but surely, you chip away at the credibility of the naysayer voice, until the scale starts tipping over, and an encouraging voice emerges.