A lot of those can be attributed to shortsighted executives who are afraid to take risks or academics who are convinced by their own limited research. And in such cases, there is almost certainly support to be found elsewhere.
So if your idea is constantly getting shot down, don't build a "I'll show them!" attitude. The best move would be to ask why they think it's a bad idea, consider it, and move on.
"The best move would be to ask why they think it's a bad idea, consider it, and move on."
Even better - ask yourself "What would it take to find out, empirically, whether this is or is not a good idea?"
Many experts deal mostly with opinions, because it's to their advantage to do so. People listen to their opinions, people often pay money for their opinions, and the whole reason they're considered "experts" is they have (presumably) spent a lot of time in the past gathering facts to support their opinions. However, if you're dealing with something truly new, nobody will have enough facts to say whether or not it's a good idea.
As an entrepreneur, your job is to find those facts, because nobody else is going to do it for you (if they do, they're the one who gets the money ;-)). So ask yourself, "How can I narrow this problem enough to build a prototype? What data do I need from users to decide whether it's worth proceeding?"
The same applies to random advice you read on the Internet. This post is an opinion - I have no hard data in here to back things up, other than my own experience. However, it's a testable opinion: the next time you face a problem, go out and canvas the opinions of as many experts as you can find. Make a prediction, based on their opinions, of how you think the problem will be solved. Write it down. Then go about collecting data and actually solve the problem. Compare your prediction with the actual approach you needed to use. If they're basically the same, I'm full of bollocks and you can ignore me. If your initial assumptions changed significantly, hard data's the way to go.
"I think there is a world market for maybe five computers."
Thomas Watson, chairman of IBM, 1943
At the time, there wasn't even that large of a market. Watson definitely wasn't panning the technology, just noting (correctly) that the time wasn't right for a marketing push.
Great ideas implemented too early aren't worth much at all.
So if your idea is constantly getting shot down, don't build a "I'll show them!" attitude. The best move would be to ask why they think it's a bad idea, consider it, and move on.