Most people experience those bad days when you don't believe in yourself as much as you usually do. Some people experience them more than others, and to some the effect is more intense than to others. God forbid, if there's a person out there who never doubts himself - I salute you.
Perhaps you've felt like it for a while now. Every day, you hear that tiny yet growing voice within that says you can't do it, that there's no point trying anymore, it just won't work and you're wasting time. Tomorrow, the voice is a little louder. Eventually it will have you brainwashed.
So before this happens - before that voice takes complete control and before you drown in the abundance of self doubt - shake yourself awake and shout to that voice:
I am [first name] FUCKING [last name] and I can do whatever the FUCK I want.
And yes, you need to use the curse words. Choose ones that speak to you. It will have a superb magnifying effect on the sentence and thus on your psyche so do not omit it. I prefer the F word but you can use whatever you want. Just use them.
The way I like to see this technique is that years from now, down the road, in the future when you'll be successful, your name will fill that sentence nicely. No one will think you have to battle with a little voice inside; people will think you own it, that you can do whatever you want, without asking permission from some insignificant and inexistent voice. People will think that you achieved the success because you are [first name] fucking [last name]. Your name will fill out that sentence completely and unconditionally.
But in order to get there - in order to finally someday fill that sentence - you need to start behaving like you already do.
When talking to the resistant voice within, add the curse word between your first and last name. The voice will get scared and hide in the shadows - and you're free to do as you please.