I never really had to give up on any of my dreams, because I always tended to keep them realistic. When I was younger, I did want to go into space, but when I learned what would be required of me to do that (fitness, flight experience, probable military experience), I quickly decided that it was unlikely for me to ever become an Astronaut/Cosmonaut.
Ever since I was about 10 my dream was to become a scientist. I've done that, so I'm happy. My goal was always to do the best I could in science, and as long as I could do that, I would be happy. Now, I've decided to study to become a doctor as well (job security + it's always nice to apply the research to patients). It will mean another 4 to 6 years of my life will be spent studying, but if that's the price, then so be it. I'm just lucky I was born in a family that is entirely supportive of my goals and have helped me to achieve them. My family is not rich by any means, but since I saved some money, and since I've always studied hard, they are willing to support me through my studies. I'm lucky. Privileged, in fact. Not everyone has the same opportunities I've had. My nation's government, the Australia Federal Government, will also foot the bill for my studies (in return I have to work for them for the same amount of time I spent studying). I've managed to achieve my dream, and I have the opportunity to pursue another dream - helping people as best I can.
I kept my dreams high, but realistic. I never wanted to become a star athlete or a rock-star. Such dreams are cool, but unrealistic. There are hundreds of millions of kids who want to be world-famous rock-stars. There's only room for a handful. Odds are, you are not going to succeed, unless you are extremely, EXTREMELY talented in a VERY specific way. As for becoming a doctor, or a scientist, or a stock-broker or a pilot - those dreams might require some luck (for instance, it's doubtful you'd be able to become a scientist if you are living in Somalia, because that nation has no infrastructure or government), but many people in the developed world CAN succeed if they simply work very, very hard. Not everyone has the opportunity, but quite a few of us do, and I'm betting a lot of people reading this forum could become scientists if they wanted to. If I could do it, I'm pretty sure anyone who doesn't have a caved in skull could do it.
So I've never had to give up on any of my serious dreams. I'm lucky. There's no other way to put it. I was born with a supportive family in a good country. I did nothing to earn that. But I'll be damned if I don't make the absolute best use of the resources I've been randomly allocated. I can't justify being in a privileged position, unless I use it to help as many people as humanly possible.