Distract yourself. Lift weights, get a hobby, get into your career or into your schooling. Work on your dreams, whatever. You shouldn't get a lick of true happiness from other people, and other people shouldn't cause you a lick of angst. Make sure your life is occupied with things to do and goals you want to achieve. Women are only one part of life. You're making a mountain out of a molehill.
Get mental help. Not joking about this in the slightest. I wasn't just clingy with girls, i was clingy with everybody. I don't shame that part of me anymore, but the roots of it was some truly horrific mental plumbing, I realized. It's not normal to need people the way we do. There's some dark shit there that needs to be brought into the light, and you need to learn how to cope with it all. This is what you should be making a mountain out of. Because when you're an old married guy and your libido is gone and yet you're still unhappy, that's when you'll seriously regret you didn't pay more attention to this part of your life. When you can get real happy with yourself, then you should focus on relationships.
Start cold approaching, hit the bars and clubs if you're 21, start finding women in your social circle, just get your handsome face out there. Even the female equivalent to us get more sex (and then scare the dudes off because they don't have to do handle their internal issues like we do!), and that's because us men have to lead the courtship ritual. Your job is to become one of those men.
Most men know how to speak to girls and have the bare mininums of game down without having to formally learn it. Guys like us don't because we didn't have strong male role models in our lives, some of us didn't even have male friends to observe this stuff from. We have to find our own male role models, and we have to learn this normal skill alone. And because we have to start from the complete bottom and struggle hard to learn this, we're even better and more well-rounded men for it. This is not an arcane magical skill. It's fucking fun
One day, a fling like this will be meaningless. One day, you might feel like fucking, call her up, and she's busy. You shrug your shoulders and simply call another girl up. If you get a girlfriend and she begins acting weird, you won't be one of those guys who'll stay with her years longer than either of you want to, because you're afraid you won't find another. And you definitely won't feel miffed about something as tiny as all this!
Game is hard work, and game is power. And this post screams how powerless you are right now. This whole situation should be a wake-up call.
Get to work, man.