You're speaking from a complete n00b standpoint where you'll suck no matter what, but since I already had some TFC experience, TF2 was easier for me to get used to than what might have been for others, but even after watching videos displaying how to be good at a spy, and reading tips, it is still extremely difficult to play as the Spy, because he is meant to handle so specific situations which don't always come so easily, and since people don't stand still, it's been difficult for me to learn to aim with the back/side stab, because I don't get as many opportunities to actually practice backstabbing than say: shooting someone with a shotgun.
You can say that you can master the Spy just like you master any class, by taking the time, but the Spy requires many sacrifices to be made as well as time, because you have to sacrifice the ability to help others on a whim, and sacrifice the ability to jump into situations. This creates less chances to actually practice spy techniques, for when I try to kill someone, I usually fail, and have to work very hard to get into that situation again.