So the plain odds are in the last 70 years there have been 100s of astronauts and 1,000,000,000's of people born so the roughest odds you're looking at are 1 in 10,000,000. But in the next 70 years we can expect 1000s of astronaut so now the rough odds are 1 in 1,000,000. Obviously being born in a first world country and perusing a background of science, engineering, or piloting can increase those odds significantly. But realistically every astronaut has been pretty lucky as well as driven and qualified. I think the quote "Shoot for the moon, even if you miss you'll be among the stars" applies here. Go for it, and apply yourself, even if you 'fail' your education and experience will come in handy with whatever career you land yourself into.