My t recently disclosed to me that I have DDNOS, which I had suspected for some time. I've done some research on it, but it looks like it is not a legitimate diagnosis in the DSM-V book. Is this true?

Can someone tell me what are the main similarities and differences between DID and DDNOS? Also, is DDNOS considered a serious mental illness or not? How many years does it take to recover? I've been in t over 10 years now, and still dont' feel finished. It makes me feel like I'm a failure! Even though my t and I work hard, and I have made alot of progress.