It is okay to hurt because someone leaves or is gone. It's actually a beautiful thing. It means that you opened up your heart and gave someone a piece of it. You trusted and you loved. That's what life is about. It's about finding those people who will get over the walls you've set and make you vulnerable. For some it is so easy and their walls are not as thick and tall.
Then there are others whose walls are almost impossible to get passed. These people are the ones who were hurt bad, who gave their heart to someone who left, but I am a firm believer that if you've opened up before you can do it again. It may be hard, but it is not impossible. So don't give up because broken hearts can be mended and walls can be broken.