How do I get less needy? By getting your life more filled. If you spend you days on your couch waiting for her to text you back, maybe there is something more than getting a girlfriend that you need to work on… So start filling your calendar with things like a job, sport, going to the park, seeing friends, visiting museums, … And remember Anthony de Mello's words, taken from The Way to Love:

If you learn to enjoy the scent of a thousand flowers you will not cling to one or suffer when you cannot get it. If you have a thousand favorite dishes, the loss of one will go unnoticed and leave your happiness unimpaired. But it is precisely your attachments that prevent you from developing a wider and more varied taste for things and people.