Other People's Opinions

Is it right to let what other people say influence your own point-of-view? I mean, not influencing what you think, but influencing how you feel? If someone says something to you, telling you exactly how they feel about a certain situation, and this is completely opposite what you think about it, is it right to get upset over it? I say no, and want to stand behind my belief that no one has the right to make you feel like crap, but it's hard not to at times. If you are so confident and so hell bent on something and someone tells you it's never gonna happen or it's never gonna work, why does that affect how you feel? It's just an opinion. It's just what an outsider has to say about something that doesn't even pertain to them, but why does it seem to have so much power me?
I like to think that I am a strong willed and tenacious person, but at times, I feel so weak. I feel like if someone doesn't approve of something or they don't share the same belief system I do, I am in the wrong. Ultimately, that is wrong too. I am wrong in believing I am wrong, because no one has the right or no one should have the ability to disprove something that they know little about. Maybe I am wrong to get this upset over something so small and stupid, but I can't help it. If you have 20 people behind you on something, and 1 person says that it can't be done, why should what they have to say about it alter what I feel about it? I have so many people behind me right now, why should I let one person make me feel like everything that I have done and everything that I have seemingly accomplished was done in vain?
I think by writing this, I am just trying to reinforce to myself that I am strong enough to not let one little opinionated speed bump throw me off the road. I know what my mission is and I know what I feel, and I am not going to let what anyone says or thinks make me feel as though I don't have a snowballs chance in hell of getting what I want. I know I may be a leaf in the wind right now, and I have no control over anything, but that unseen force will keep me dancing through it all.