I have been fatigued and it actually doesn't matter what stage of an individual duty you are at, it could be on the taxi out of the first sector at 8am and you can still be mentally ill equiped for operating. A duty can appear to be a good one on paper and you are not even capable of recovering well enough to do it. Eg you can have a day off with 10 hours sleep and not be sharp enough to operate a 10am sign on sector.
In my case it built up over a twelve month period and was due to a combination of household circumstances ( new baby, two bedroom apartment, no sound proofing) and poor rostering ( lates into earlies and back again every week perpetually). One night sleep didn't even put a dent in it. It took at least six months of getting proper uninterrupted sleep before my usual mental processing capabilities returned. As an aside, so did my positive outlook and ability to remember not only what day of the week it was but what we had planned on that day.
For me it was the circadian disruption that got me more than the lack of total sleep hours, but everyone is different.
The rosters that you flew GSXTY should be illegal and the head of FlyD should have to defend his/her oversite of the rosters produced leading up to the crash ( I won't say accident).