Immerse yourself in a new passion...and you might just find a new passion.
Seriously, get on with doing something you like (a hobby) or need to (studies or job). Not only will you begin to occupy your mind, you might have your eyes opened to a new friend.
And when all else fails, borrow a friend's kid. You'll make your friend a very happy camper by giving them an hour or two break. Babies, in particular, and little kids in general are also "chick" magnets...especially of the type that have any interest in a long(er) term relationship. And, maybe being around a little person...you'll learn a little bit about yourself too: are you cut out to be a parent yourself? Nothing will make you "grow up" faster yourself than having to look after someone much smaller than you. Finally, even as frustrating as they can be, kids can really make you smile. Especially when you get to give them back.
Not sure of your preference of music style, but there are many songs about heartache and missed "chances." My current favorite that might apply to your situation is called This
by Darius Rucker.
Good luck, mate.