Sometimes you will play AA and win, other times AA will get cracked.  Why this happens is because of variance.  It can strike at any time with any hand no matter how good your odds are, you can still lose to variance.

How to beat variance is simple.  Learn as much as you can about poker and that will help immensely.  Also reduce the chances for variance to get you by playing only premium hands from good position and bet them agressively.  Finally, don't forget to fold way more hands than you play.  That way you don't get victimized by variance and will probably end up lasting longer at the tables.

Hope this has helped your game a little.  Good luck.