Every single one of those stories are "different" and "complicated". We've all been there before in one way or another and some point, and some of us still haven't completely gotten over some people.
There's a girl I've had an irrational crush on for years, and to this day she still makes me self-conscious and weak in the knees when I see her. I talk to her when I see her, and it crushes me that she will never know the joy that is the relationship that we could have had, were circumstances different. I'm in a happy relationship and I get lots of female attention as a performer and musician, and yet this one girl still has such a profound and inexplicable effect on me. I accept that.
Attraction is not a choice- but how you respond to it and what you do about it most certainly is. Acknowledge and accept how you feel, and then ask yourself- what do you really want to be doing about it? How would she respond, or react? If you dare to listen to yourself, you'll find that you already know the answers.