This might just be the hardest concept for poker beginners to understand. Or accept. Hell, I know plenty of experienced players who have trouble accepting it. I've blogged about Variance off-and-on over the years, but it still bears repeating: just because you are a good player does not mean you will win at this game-- at least not over the short term. Poker may be skilled based, but only over the long run. The Guru used to teach to his class--quite emphatically--that poker is 100% skill-based, but what he meant was it was skill-based over the lifetime of a player. Whether we like it or not, Lady Luck plays a significant role over the course of a session, a week, a month. You can be smart enough to get all your money in preflop with aces against deuces over and over, but luck is going to determine if your Rockets hold up or not against the Ducks. Over time, Aces will win a significant fraction of the time, but on a hand-by-hand basis there are no guarantees.
It is important to understand that just because you are currently winning at poker, it does not mean you are actually playing well. Conversely, losing does not necessarily mean you are playing poorly, either. Variance can greatly skew your short term results. You have to evaluate all your hands--both winning and losing--to determine if you're making good decisions. And that's all you can do. The results don't really matter. Why? Because of Variance, my friend. Accept it.
All-in for now...