In my personal opinion, I would go through all the characters, learn their moves and mess around until you find a character that you really feel comfortable with, then go with them. As for your online losses, whereas it’s true that some characters are stronger than others, most of a match comes down to player skill. So…don’t blame it on Sakura, because she’s honestly a pretty good character. You just need to practice and learn more. I still get beat up by some really good Sakuras. And you may not want to limit yourself to only one character. A lot of people have multiple characters to use as somtimes you’ll run into your characters counter character and you may want to pick someone else who has a better set of moves to use against them.
As for your non-safe jump-in’s…well, the description you gave should be a dead giveaway as to why you’re losing. =P They’re not safe. A big problem for beginners playing online is that there are a ton of medium skilled and high skilled players online, so when a beginner comes in, they usually get bodied so bad that they get discouraged and sometimes even just give up on the game entirely, but stick with it, and be patient, and learn from your mistakes. One of the benefits to going through the game and using all the characters is that it gives you education on what their moves are and when they can use them, so that when you are fighting against them, you know when you can jump in and when you can’t, what distance is safe, what moves to use to control the match with, what moves you can punish and when, etc. The game is really a very precision oriented game, and again, that kind of thing requires a crap ton of patience to learn, but I promise, when you do start learning and eventually start winning, the joy you get out of it is totally worth it.
This game to me, is both, the most frustrating and most rewarding to play. I want to punch a hole through the wall or even my own head sometimes, but other times, I run around the living room screaming like an rtard, cuz I just outplayed someone else. Good luck man.