You should read: https://www.englishexperts.com.br/forum/quando-usar-this-ou-it-t8967.html
"This" is a demonstrative pronoun.
"It" is both a subject and an object pronoun.
It would be better if you had said, according to Stylistics studies.
"I really don't know, but if you want to do it
, just do it"
In order to comprehend better the difference between "it" and "this", you should access:http://www.grammar-quizzes.com/it-this.html
Enjoy your studies!