You can get to be great at most games if you practice enough, learn the levels, and put in the time. Starcraft II is not one of those games. There will always be someone better at it than you, so get over it now. That being said, because the pinnacle of Starcraft dominance is so high, you'll always have something to strive toward, insuring that you spend even more time playing. You should probably just break up with your girlfriend now; you don't want to deal with that in the middle of a match.
You don't need a Xel'Naga watchtower to see the profound strategic effect that the new map elements are going to have on Starcraft 2. Destructible rocks provide a false sense of security. Tall grass allows you to ambush your foes (and to find Pokémon). High yield minerals allow you to produce even more pylons. Elevation gives your soldiers a new tactical advantage. Finally, Xel'Naga watchtowers let you see your impending doom from even further away.
If you mod it, they will come. Starcraft had mods ranging from classic tower defense to Dragonball Z. With the new Galaxy Editor, Starcraft II looks to build on that tradition. So once you're finished with the campaign and bored with getting beaten online, you can make your own game. Possibly that Jane Austen mod you've been thinking about. Personally, we want to see someone tackle "Fear and Loathing on Mars Sara." The gauntlet has been thrown down Dorkly readers.