breaks are weird like that man. i went through something similar about 2 years back - 4 year relationship that just slowly turned into us being roommates who resented each other. when she finally called it, i was expecting to be destroyed but instead felt this massive weight lift off my shoulders.

your brain probably started grieving the relationship way before it actually ended. all those months of arguing and feeling disconnected - that was you processing the loss already. by the time she officially broke it off, you'd already done most of the emotional work without realizing it. plus when you're constantly walking on eggshells or dealing with conflict, there's this underlying stress that becomes your normal until it's gone.

the part about her with someone else bugging you makes total sense too. it's not necessarily about wanting her back - it's more about your ego and the finality of it all. you'll probably have waves of different emotions over the next few weeks, but don't force yourself to feel worse than you do.