If you're having issues with other specific characters or against characters people you play against a lot main, try playing *as* that character for a bit.
When someone else is wiping the stage with you using that character, it's impossible to see their flaws, but if you take them out for a test drive yourself you'll notice the gaps and windows in their movement and attacks. Which moves have lots of startup and endlag, which moves can't hit behind or above them. What the character wants to do and how they want to come at you... and also how they don't like to be approached because it's hard to deal with.
Zelda is slow and if you play her you'll notice where the lags in her moves are so you can hit her during those windows. You' also realize she doesn't like people up close in her face or below her since a lot of her moves can't really deal wtih that save for maybe one or two.
But I'd say go online, play as Zelda and save replays of other fighters tearing you to shreds. (lol, sorry but we've all been there)
Then watch the replays and see how people approached and got in on your Zelda and try to apply those strategies to your mains.
As far as Mewtwo, he's a lot faster than Zelda and can rush her down without having to get too close. Remember that you can reflect her phantom, but not her fire ball. If she sends the fireball, be sure to fully shield it, then dash in. Shield again if she throws another one out.
If you see the phantom, just jump over and attack her from above. You could always fire off a weak shadow ball (because if anyone so much as sneezes on her while she's charging the Phantom it'll crumble), but always jump over the Phantom, never try to shield it.
If you see the green wind from her teleport move, just hold shield until she reappears. If she tried to hit you with it, you'll block it and get in a free hit.
Other than that, Zelda can't really come at you with other normal moves since Mewtwo outranges basically all of them. If you know your opponent's moveset from playing them for a bit, Mewtwo is fast enough to rush in on anyone, and he's able to reflect projectile spammers once you know how those attacks work also (Confusion has start up so you have to know and be ready for when a character will likely shoot something at you).
