Mmkay, couple of things. First, there are definitely folks on here who have dealt with fatty liver, so hopefully they will chime in here.
Condolences about your mom—that is a rough thing to go through, losing a parent, and I can empathize.
Now ... you were diagnosed with fatty liver at the end of November, so like only two months ago now? Do you really, honestly think that your condition would have progressed to NASH in only two months? Think again. I don't know how old you are, but liver disease just does not progress that quickly. As far as the pain in your right ribs, well I think you're adding together two things that aren't necessarily related and coming up with catastrophe. That pain could be anything. Pain resulting from inflammation and/or enlargement would bring with it a plethora of other symptoms, I'm sure. There's NO WAY a lack of immediate action on your part to lose that weight would have caused your condition to progress in such a manner. That right side pain could very well be muscular or back related. I had that, too and, combined with some nail symptoms, was convinced I was in liver failure—I wasn't.
I don't mean to sound insensitive but you mother just died of liver cancer, and now here you are, only several weeks later, fearing the same thing. Mmm, I'm not sure that's a coincidence. Neither is the fact that today is the day you decided to google your condition and now you're scared sh*tless.
Listen, do the work the doctor has put forth to you and lose that weight. 5 or 10 percent is not a lot. Also, not meaning to sound condescending, but try and find someone you can talk to about your mom's passing. That's kind of a big thing and the better you can process it now, the better off your anxiety will be. My dad died in 2005 but I didn't reach out to a therapist until 2011, and I wish I'd done it sooner.