I do not understand how they go from you are the love of their life to so cold so fast
they dont.
one of the hardest parts for me, in detaching from the wounds of my breakup, was realizing that my ex and i were on very different pages, the more the relationship went along, and especially the closer we were to a breakup. it didnt feel that way at the time. it rarely does.
the fact is, the two of you had experienced at least one breakup (a relationship killer in most cases). you were together for at least 3 and a half years (a bit longer than my relationship) and as you indicate, things were downhill after year one...even more so, approaching the breakup.
the hard part is that she likely had grieved the relationship, partly or completely. the harder part is that you likely had as well, but you werent done, and you still had hope. its always confusing, hurtful, and empty feelings to be on the receiving end of that.
it likely will hurt to look at your relationship that way: as unraveling for a long time, but it will help you to answer a lot of your questions, and it will help you detach.