dear sara, in here 'it' mentions sticking to a diet, in other hand 'she finds sticking to a diet difficult'. hope it helped you

Hi Sara In your sentence, it is what we call an empty or dummy subject (or object). It’s empty because it doesn’t refer to anything in particular. I think it’s going to rain today. It’s getting late now. She doesn’t like it when I am quiet. Your sentence basically means the same as “she has difficulty in sticking to a diet”

IT just means to stick to a diet - She finds sticking to a diet difficult. We use IT as a pronoun so we can say what the adjective is before we say what the often long object phrase is. By doing it this way, it's easier to process the information and it can make it sound more interesting. She finds IT difficult ... - we know that WHAT she finds difficult is going to be said next and we have a sense of anticipation now. If you have a very long object, it is clearer why it makes it easier to understand: I find that the introduction of the new rules concerning the way the tax laws will be implemented is going to be delayed by a year interesting. I find IT interesting that the introduction of the new rules concerning the way the tax laws will be implemented is going to be delayed by a year. The second sentence is much easier to understand just by the addition of the empty pronoun IT. In the first sentence, by the time we find out how they feel (interested) we have forgotten what they are talking about.

Grammatically, "it" is used here as a pronoun. As such, "it" is used as a noun that refers to something else that is easily identified. Your sentence is equivalent to the sentence: "She finds [to stick to a diet] difficult" So "it" serves as a placeholder for "to stick to a diet". In your sentence, the principal phrase is "She finds it difficult" The remaining words "to stick to a diet" simply identify what "it" is. Whenever you use the word "it", you must make it possible to identify what "it" refers to.