The thoughts of others add or detract value from your thoughts … only to the degree that you allow them to. When someone tells you that you can’t achieve the desired outcome, a goal, question them as to why they think that’s so. If you give their sentiments validity, question yourself as to why you’re thinking in such a manner.
The point is, everyone is influenced by outside stimuli, but we’re driven by our internal thoughts. Protect the thoughts that motivate you to move ahead. don’t be deterred by the naysayers. The thoughts that promote a ‘can do’ attitude in you will be the guides that lead you to more winning negotiation outcomes.