I like that you're trying to get better.
Ahhh. As a player, I train by, well, playing. I play people that are better than I am. They teach me what works and what doedn't work. If you play better people then they'll punish your mistakes. It's easy to get away with stuff on scrubs, but it's hard to even kand hits on good people. They predict, read, and analyze you within a minute of the match. They'll pick at your bad habits, and punish you. Just play people better than you, and you'll play more safe.
getting better with your character requires extensive knowledge of him. Frame data, range of each move, when to use what. that comes from matchup experience. If you play good MK's all the time, then you'll know the right thing to do, but if you play jiggz once in a blue moon, you'll probably have to adapt.
It takes alot of time to get better.
Know your character.
Know your opponent.
most of all
Know yourself.