The word "it" requires an antecedent, that is, something that you have just recently mentioned. The word "this" can be used in the same was as "it", with an antecdent, or it can be used when you are identifying the object by holding it up to view, pointing it,
That is, if you walked up to someone and, as the first thing out of your mouth, said, "I am sharing it", their natural respone would be, "Sharing what?" Even if you were holding the object in your hand and held it up in front of them as you spoke, I think the natural reaction would be, "You mean you're sharing that?"
If you identified the object in question first, then "it" makes sense. Example, "I have a new book. I am sharing it." Obviously, then, you are referring to the book.
On the other hand, if you walk up to someone with a book in your hand and say, "I am shring this", "this" would be understood to refer to the object you were holding. Likewise if you pointed to the book where it is, say, sitting on your desk, and said, "I am sharing this," people would understand you to mean the thing you're pointing at.
You could say, "I have a new book. I am sharing this." But that would generally be considered a somewhat odd usage unless you had just distinguished it from some other things you might share. Like, "I have two new books. One of them is about horses. I am sharing this." Then we would understand you to mean that you are emphasizing that you are sharing the book about horses and not sharing the other book.
If you walked up to someone with no object in your hand, not pointing to anything, and without any introduction, both "I am sharing it" and "I am sharing this" would leave the listener wondering what it is that you are sharing.