Why am I thinking about her, not the other?

So, 2 long stories cut short.
I dated a Mormon girl for 5 years on and off. Things at times we're amazing, and before she left on a mission, and our relationship crumbled. We talked about getting married, looked at engagement rings and had tentative plans for a wedding and honeymoon in San Francisco and Disneyland.
Anyway, she broke things off during the mission and everything crumbled. We weren't able to salvage the relationship. And I struggled with it.
But within a month, I met someone very nice and we dated for approximately 8 months. We both got out of very painful relationships and we meshed very well. She was busy with school, but we spent the better part of a month together during winter break. We had planned at some point, if things went that long. That she always wanted to go to San Francisco, and I always wanted to go to Disneyland. So we agreed to go with each other.
Anyway, that relationship ended. It ended worse than the Mormon one, and we aren't speaking or anything.
I ran into the Mormon and felt things could work themselves out. But nothing. I let it go.
I joined my family on a trip to Disneyland, and I thought it'd enjoy myself, and I have. But I have been thinking about how I could have done this with the girl I just stopped dating about 2 months ago.
What confuses me is I've been caught up with the Mormon, and this was about the time we were supposed to go anyway and be married. But instead of thinking about that. I'm thinking about the other girl.
I'm confused. I was caught up with the Mormon, and she was in my mind more.
Why am I thinking about her, not the other?
Add Opinion