Firstly I'd like to say don't get hung up on grades, just make sure you enjoy what you're doing. Sounds a bit gay and a bit cocktalkish, but I think everyone will agree it's the right attitude.
Secondly, grading stuff is seriously hard, the only way to be able to do it is to just climb as much stuff as often as possible. I've been bouldering for about 18months and I only just feel that I can grade stuff. But you'll always find people who'll disagree with you, so don't worry about it too much. They're only a rough guide anyway, one thing might be piss to one person but impossible to another, regardless of strength.