Use either as appropriate.
THIS ...f you are discussing a specific subject, thing, idea, etc.
IT if you are discussing a specific subject, thing, idea, etc.
English is a very flexible language. Do not be so quick to presume that some kind of rule prohibits flexible usage.
It is a nice day. This is a nice day. It is a nice coat. This is a nice coat.
It is a love story for the ages. This is a love story for the ages.
It is a disaster. This is a disaster. It is true that I love you. I love you; this is true.
This is not the right way to the airport. It is not the right way to the airport.
It is exactly what I would do. This is exactly what I would do.
It is yours. This is yours.
(Hope that helps.)