It's funny how your brain needs to reorganize your thoughts when something big in your life changes.
When my Pap Paw died in September it took me several weeks to realize that he was really gone. I had to re-frame everything about parts of my life. At breakfast, lunch and dinner I would remember and grieve. I would think of summer and think, "I'll have black-eyed peas and fresh corn from the garden," and then suddenly have to remember that he was gone... A sale on Bluebell sparked the usual thought of "If Pap Paw sees this he'll buy all 5!" Each time I would have to remind myself, "No, he's gone. That is over now."
The same is with when you break up with someone. You have to go through each part of your mind and re-label "boyfriend" as something else; he is now ex-boyfriend, my ex, or just his name. I run across things and think "I'll have to show/do that with/talk about it with him... shit."
I wonder if my brain is physically re-organizing itself, growing new paths, forcibly pruning the incorrect ones. Maybe that's why it's so hard.