Both are correct. Although not completely natural. I would change them as follows: You have been specially selected from *among* thousands of people. (amongst is quite formal) If I hadn't enjoyed *doing* it the first time, I wouldn't have done it again.

Hello there, Well about those 2 sentences, I should say that the second sentence is correct in my opinion, as a conditional sentence type III. "If + Past perfect + Would have p.p"\ But about the first one, I'm not 100% sure, although it sounds to be correct. : )

1 is fine, though I would use 'among', which is equivalent to 'amongst' but I would say was slightly less old-fashioned. 2 is fine, but slightly clumsy; it was it, not the time, that you enjoyed. A concise way of saying this would be: if I hadn't enjoyed it the first time, I wouldn't have done it again.