Your resolve is admirable.

The last part of that first line is a little confused. You said, "Definitely/surely, if I want to get better, that's no good" [except I don't think you can say なりたければ; I think that "-kereba" doesn't join on to the -tai form... not sure though...frankly, I've only ever seen/heard -"kereba" followed by the negative "narimasen" e.g., 勉強 しなければ なりません　I must study)]

If you want to say "if I want to get better" you could say よく なりたい なら、

But your それはだめです makes me think that you were intending to say something more complicated. Something all dependent-clause-y.

Oh, maybe you wanted to say "that's no good," because of the previous sentence about not practicing much.

But the "zehi" won't work then, because that "zehi" is only used when you want to encourage someone to do something pleasant, or when you yourself are looking forward to something or want to do something. i.e., ぜひ　行きたいです。 is okay but ぜひ　いきたくない。 is weird.