Seriously, I just went from playing with level 1-10 players to 10-50+ players. I used to be able to do pretty awesome, sometimes be at the top of the scoreboard, but now... I can't do anything. I'm sure it'll get easier with practice, but man, what a buzzkill it is to be pretty good at a game only to realize that the actual game outside of novice is significantly harder.
It's good though; you should find that you get better a lot faster now that the pool of challengers has opened up. Although if you find yourself continuing to struggle, I'd suggest focusing on one or two classes to specialize with for a bit.
Also, it's never too early to get in the habit of going for mid-airs. A major mistake most new/low-rank players make is assuming that mid-airs are unreliable to the point of giving up on them. If you can spare the ammo and expect to reload before your opponent hits the ground, failing to go for a mid-air and settling for ground-pounding (a valid tactic, but one that will only ever yield moderate damage in most cases) can hurt you in the long run. On the other hand, get used to going for mid-airs early on and you'll find yourself getting the drop on people 10-20 ranks ahead of you in many games.