Confidence is a two-edged sword. If overconfident, pride will lay you low. However, if one lacks assurance, they lack the drive to succeed.
Belief in oneself must always be honest. Lies cannot stand against the billows of scrutiny. At the slightest breeze, it will all fall down.
Those who lie, inevitably lie to themselves. And they start to believe their own press. Soon, they are ensnared in the web meant for others.
Believe only in that which is demonstrated. Show what you can do, and the naysayers will be silenced. Or their words will fall on deaf ears.
Accept constructive criticism. If someone is trying to tear you down, it will be obvious. They will have nothing good to say.
Once you know your abilities, what they say becomes meaningless. Listen to them anyway. Let it stoke the desire to succeed, and leave them behind still grumbling.