Me with estimations:

*enter the supermarket
*take a small basket because I need just one or two things..
*realizes I needed more things, basket get full, me getting tired.. oh shit..

Boss asks some changes
-ok boss, one hour at most, it's just adding few lines..
*then I realize that it's not that simple

100% with you on this. I can't estimate for shit even though i have been a developer for years!

A trick someone taught me was to take how long you think something will take and triple it for any estimates. Quadruple it if its bigger/more complicated than usual.
On the offchance that it only takes your original amout of time, you now have longer to do it properly, try out new technologies, improve test coverage etc