Jakeyjake, it would make far more sense to jump in with SF x Tekken than UMvC3.
UMvC3 online is mostly a pool of players who are experienced and will stomp the shit out of you. Even on release day, the level (for online) was fairly high because it was mostly MvC3 veterans who decided to buy it, so they already knew the game engine, how everything worked, etc. There weren’t many players completely fresh to fighting games who decided to jump in with UMvC3. UMvC3 is a game where you can get reeeeally fucked over anyway because there’s a lot of hardcore, abusable shit to fight in that game. If you don’t know what you’re doing, it can be really, really hard finding your feet.
SF x Tekken will have a flood of new players when it’s released and will likely have a lot of players who aren’t that familiar with fighting games for the first few weeks, so that will be your best for deciding if you’re cut out for the genre or not. If you have any buddies who get the game, even better, but the best thing to do is play those on your level. Even if it’s random players online you had a close game against, send them an invite/friend request and do player matches against them. It’ll come together naturally.
I honestly think the problem you’ve had is UMvC3 has an unusually high level online, due to the high percentage of veterans and low percentage of new players coming into that game today.
In any case, as with all fighting games, the best way to learn is to find human opponents on your level and take it from there. Practice in training mode is important but it should be something you naturally gravitate towards because you’ve fallen in love with the genre and want to get better. You will not discover your love for fighting games in training mode.