I'm curious to know how early you were exposed to hearing Mandarin sounds (from birth?) and whether you spoke Mandarin at all with your family before you moved to Taiwan.
I ask because one of the toughest things getting started for many foreigners learning Chinese is being able to distinguish the tones when listening and speaking, and AI read some report of some research that said that early exposure to hearing another language (I think at under 12 months old) allows you to distinguish the sounds that occur in that language, even if you only learn that language later in life.
If you went from zero to fluent in 3.5 years, that's awesome. I can imagine the second half being hard but doable. I can't imagine how you got through day 1 and month 1 at all!