Just some tips that could possibly help you out. Take control, know your match ups, surroundings and what your opponent is doing. If you focus on yourself, you'll probably end up losing. After all, you're controlling your character and you should know what you're doing, so keep a look out on your opponent instead. As for nervousness, I don't really know how to overcome that. I've just fought many different players that I don't get nervous as much as I used to.
Fighting tougher opponents will make you better. It just takes time. Also, I don't know about you, but online and offline matches are different. You can probably beat me offline. I mean even though it was laggy, you still managed to do pretty good. Personally I prefer real time over wifi. C: