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The second 私 is superfluous: you already mentioned in the first part that the sentence applies to yourself, so you don't need to repeat it. In general, use 私 as little as possible to avoid being overly verbose - only use it to emphasize that something applies to you specifically.
The だ should be left out: if a sentence in the short form ends with an い-adjective, the adjective itself functions as the verb. (For nouns and な-adjectives, だ is needed however.)
Finally, 練習です is invalid: you can't "be" practice, you "do" practice.
So the corrected sentence would be:
私は日本語で書くことが難しいから、毎日練習します。