It doesn't matter if you were married for fifteen years, or if it was a one-sided crush, making the decision to move on...& actually stick with it...is an absolute bitch. One that does not get any easier with age.
Let's face it though, if you weren't meant to move on, you wouldn't have a reason to. For me, when I start to feel like I'm 'chasing' someone, it begins to become clear to me that this person, who I probably adore, has no place or desire for me in their life. I have to make myself stop texting/Facebook messaging every time I want to talk to them. I have to recognize the reality of the situation, stop trying to make a place for myself in their life, & stop giving them a place in mine.
Moving on is painful...physically, mentally, emotionally, psychologically...every single fucking way you can think of. Hitting yourself in the head with a hammer will sometimes hurt less. I have a horrible habit of using alcohol as a crutch in these instances...which has lately caused me to drink too much on an empty stomach and spend 2 hours babbling at my friends in such a way that the only 2 words they could understand were 'that boy'...and also showing them my cleavage...but I digress.
I do not advocate excessive drinking to the point of harming yourself, hard drugs, or hurting others as a means of aiding in your moving on journey. You know what falls under the 'hurting others' category? Rebounding. It's cruel to start dating before you're ready. Somewhat cruel to yourself, and bloody fucking cruel to the person you possibly cause to start developing feelings for you. This is why I treat my friends who try to fix me up with their recently divorced/'broken-up-with' friends or family like they have lost their goddamn minds. You're as bad as fucking terrorists. Go sit in the corner, and think about what you've done. Don't turn someone great into a rebound mess.
It took me around 15 years to completely let go of the person I thought was the love of my life. I had to finally see that my place in his life was entirely unflattering, and far far less than what I wanted or deserved. I finally let go after spending Spring and early Summer of last year in the most godawful, deepest, darkest funk of my ENTIRE life. I knew I had to push myself through it (I even initially said no [not now, rather] to meeting up with a guy I really wanted to see...a whole other story). When I came out of it, I was done. I had let him go completely...no place left for him in my heart or my life...& there never will be again. The good thing about me moving on, when I'm done...I'm done for good.