What exactly is the difference between <verb>-てしまう and <verb>- [切]{き}る?

Question

I've read that both the ～てしまう and ～きる (18th meaning of 切る at http://www.csse.monash.edu.au/~jwb/cgi-bin/wwwjdic.cgi?1MUE%E5%88%87%E3%82%8B) forms are used to signify something has been finished/done/completed. If so, what exactly is the difference between the two conjugations (how do we decide when to use which)? example: 1) もう荷造りしてしまった。 2) もう荷造りしきった。

Answer 1

しまう - completion (and ending up in a certain condition) or indicating strong feelings such as regret or remorse.

もう荷造りしてしまった。- I packed my bags (and it is done!).

ケーキをすべて食べてしまった。He ate the whole damn cake! (Anger? Impressed?)

タバコをやめてしまう！- I'm going to give up smoking! (determination)

きる - to do something completely, thoroughly, sufficiently, strongly and or, borrowing from the actual meaning of 切る, an idea of severance - discarding or giving up

もう荷造りしきった。- I packed my bags (speaker is determined to go or packed to cover any contingency)

離れてからも彼女のことを思いきることができない - Even since we broke up, I can't stop thinking (cut off my thoughts) about her.