“The only thing you should doubt is doubt itself”
This is one from Bobby McGee, a multiple Olympics coach (I think 5 times) and is a master of the mindset stuff.
His tip when you doubt yourself is to continue the conversation. When your mind questions your own ability, seek proof from it. “What makes you think you won’t finish this race?” That forces you to come up with proof that supports your initial thought. Think of it as the conversations between the voice on the left side versus the one on the right. Be prepared when you argue with yourself to win the win argument. Have lots of ammunition.
Keep yourself accountable for the crap you put in your own head.