*I'm very ashamed to say that in my early 20's when I was dating another man I did act on these crushes and cheated on my pervious boyfriend a number of times. The thing is that if any of these men ever actually wanted a relationship with them I ran a mile! The ones who blew hot and cold or rejected me would send me into over drive and I'd become highly limerent of them and would fantasize about them for years but only ever about the point where they fell in love with me.
Like a previous poster said, I have low self-esteem and I seek validation from men. In my fantasies my main thrill comes from imaging their desire for me. Its pathetic actually but this fantasy gives me both an imaginary sense of validation and the emotional thrill and high of early falling in love, even if it isn't real.
It could be seen as harmless but I think it does erode my peace of mind, it distracts me from my other goals and it is a for of disrespect to my partner because there are times when I am with him, having a lovely time but secretly imagining that I'm with someone else, which is so unfair, especially when it is DP giving me said lovely time!*
Oh my goodness, are you me?! Like this, word for word could be me!
I have cheated I most of my previous partners because ive been bored/ unhappy in the relationship.
My current DP (fiance) is amazing, literally amazing. Kind, funny, hot, the sex is great etc etc yet since the day I met him 4 years ago I've had constant crushes on other men.
I get so angry at myself. It's definitely due to my low self esteem. Without sounding like a complete knob, I'm attractive ( I have modelled through most of my adult life) I get constant attention from guys and Iove it. It makes me feel good, validates me. And my god, on the rare occasion someone isn't interested in me, I become obsessed 
So no, OP. I'm 30 and I've been like it since I was 18 so you're definitely not alone but I don't know what the answer is!