StarCraft II's matchmaking scheme is excellent. And the league/division system has an almost MMO quality to it. Progressing up in rank within your division feels like gaining XP.
There is a downside to this: Rank Fear.
See, unlike an MMO (or at least, WoW), you can lose rank. Each game is therefore zero sum of sorts: to gain rank, I must defeat my enemy. And if I fail, I lose rank.
That's all well and good. But improving at playing the game requires experimentation. And experimentation means that you're going to lose.
If you're trying to put together a new build, it's going to fail. A lot. If you're trying to find a good timing to expand, you're going to get caught and killed. A lot.
Back in the days of Battle.Net 1.0, this didn't mean much. Oh, you got a loss on your record, but what did that matter? But in 2.0, with your division rank on the line, are you really going to expand into a rush just so that you can have practice trying to fend it off?
But what happens if you don't? Then your opponent dictates your play. If you don't know how to fast-expand against an early-aggression build, because you've always scouted it and took the safe option, then your skills are going to be behind. You're going to play it safe.
And if there's one thing StarCraft is not about, it's playing it safe.
I have a tendency to be a timid player. This is just my natural inclination; whenever things are going wrong, I tend to fall apart and lose. To counter this, I force myself to play aggressively. I try to force myself to attack, even sometimes when it's not safe or even prudent to. In order to make myself expand faster, I simply say at the start of a match that I'm fast-expanding this game. And I stick to it.
Yes, that means that if I scout early Barracks+Reactor pressure that I'm pretty much hosed. But what if I could fend it off? I'm not that good at any skilled micro (focus-fire is about as good as I get), but what if I was good enough to hold my expansion against early pressure? And what if I could learn to do it consistently? I'd be a better player for it.
Remember: it's just one match. One game. So what if your rank goes down for awhile? In a week, you'll have that rank back plus some, once you've learned how to fast-expand into anything. Alternatively, you may have learned that you can't fast-expand into anything. This is also a valuable lesson; it teaches you to focus on something else where your play is deficient.
My point is this. Experiment. Try things. Try things a lot. Don't be so afraid of losing one match that you're unwilling to improve and expand the tools in your répertoire. Because that's the only way you're going to be good at StarCraft II. Fear is the path to rank stasis.
StarCraft II is not an MMO. Your character doesn't get better simply by winning, because there is no character. There are no stats, no loot. There is only you and your skills. Leveling yourself up requires taking risks, and taking risks means losing.
Do not let your fear of losing now stop you from improving your game.