Newsflash: people change.

(I did read the update, and I'm glad about it, but it's not a given)

I agree it is (was) a horrible situation, but sometimes it might happen because - did I say it already? - people change. Sometimes for the better, sometimes for the worse, sometimes they become just different.

Believing "S/He will love me forever in the same way and won't change her/his mind" is believing in fairy-tales and unicorns, and it means one is not prepared when ("when", not "if) the change arrives.

This reminds me of the oh-so-common situation when a couple is all fireworks and lovey-dovey at the beginning, and they think about the marriage and say "We will make love all the time" or such... and some years later she is "Never in the mood" or even "Don't touch me!" (sometimes genders are reversed; same misery). :-(

TL;DR: The future is an hypothesis, never a certainty. Don't think promises are a guarantee.