You can say 1) 'I have fallen into great sadness' - You are saying it happened in the past and you are still in that state 2) 'I have been falling into great sadness' - Which means that this state of sadness is increasing, you started falling into it maybe a few days ago and it is getting worse. I hope you are not actually in a state of sadness, that it is only a sentence to study English. But if you truly feel so sad, please cheer up. Think of all the good things that you still have, and give thanks for them.

** I have fallen into a great sadness been is unnecessary