Marth is a difficult character to use. He has a sweetspot at the tip of his sword that's pretty hard to hit. The sweet spot does a ton of damage and knockback, but the rest of his sword is pretty weak to balance it. If you can't hit with it consistently, I'd suggest trying out Lucina.
You don't have to practice to get good. In fact, I'd say that practice you're doing isn't doing anything to make you better. If you want to get better, think critically.
1.) Watch videos of or fight against someone who's better than you
2.) Pay attention to what, why, and when they're doing things.
3.) Try to play with that information in the front of you mind.
4.) It'll give you better fighting sense in general and help you develop good strategies and habits to mix into your natural playstyle.
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