Ex's are so happy and in love, whats wrong with me

9 people that I have slept with and/or dated are engaged or living with a guy, and 3 more are in serious relationships with a guy. That is only what I know of. How can 12 people I used to date be so happy with there life, and yet I find myself feeling the complete opposite way? I guess I have to realize that I am just not what any girl is looking for, and girls just date me for fun or to fill a void, or because of some other reason, but over time they realize they want to go another direction in their life, and I get pushed to the side. I never get forgotten, I just slowly get erased from their memory.