Even the most talented and capable people will be challenged by pessimists. Don't automatically assume that others are correct with they try to dissuade you.
We need to be open to objective feedback. Perhaps the questions about our abilities or qualifications are valid. On the other hand, the other person's negativity might not reflect our actual abilities and potential.
Perhaps it just reflects his own self-image and mindset. When you have realistic self-confidence, you have a better chance of success.
Only by trying do we gain more knowledge of what we can and can't do at this time. If you have pure motives for what you are trying to do, it is wise to try.
The greatest people have become great because they weren't stopped by people who did not share their vision. Many times people have had to make many attempts before they accomplished what they had hoped to accomplish.
The deep conviction that they would reach their goal led to tremendous success.
(Self-confidence, chapter 20 – Artscroll Publications)