2nd mistake was packing too much stuff. Fixed a week into my trip in a Post office in San Sebastian by stuffing 2/3 of my backpack into a cardboard parcel and posting it home.
Pack light and then throw out half.
Seriously, I just came back from 3 1/2 weeks in Asia with a backpack with a weight of 11KGs (15, when I returned). I was never short on anything.
This may be slightly different when you have a weighty hobby (scuba diving, or so), kids or go to colder places. But in general my experience is that most people pack far too much.