Hi niceguy, I know it's got to be really painful for you right now. Feelings that strong don't just go away or vanish however much you "logically" tell yourself anything. And everything can theoretically be "just fine" in life but if something's missing..........
But while the pain is natural now, given time, however hard it may be to believe where you're at, the pain can ease, maybe slowly but...........And if you can gradually allow yourself to believe that there can be someone just as important in in your life.....maybe no-one will replace her but they can be equally important and give you what you need/what you want in life too without the pain you went through with the past relationship.
I know it probably won't really help that much right now but at least you know in your heart that you made the right decision in finishing the relationship. And just in doing that you've already made big steps in allowing yourself to move towards a much better future.
But for the rest....really do feel for you, and here if you want to talk/want support whenever.
Alison