What can I do? Will I ever be able to go on happily?

I am a very lonely person. My first love was an amazing relationship. We were in love. But they left me without any explanation. My following relationship was amazing as well, however that person broke my heart badly to. Every time I have ever opened myself up to somebody in a romantic way, I have been broken into a million pieces. Not a normal break up where things are understood and the two talk about it, but the terrible kind where everything is not said, and I am left with nothing but questions. Also, following the relationships, I go from having someone, to having absolutely nobody, and being completely alone and so lonely. I constantly hurt, in such a way that it is almost physical pain. The only time I can escape the sadness and loneliness is when I sleep. I'm afraid to even attempt another relationship because all I have known from them is getting my heart shattered. But I really yearn for companionship and for someone to care for me. I want it so bad. But I don't know what to do. I feel like there is something wrong with me, that makes people not want me. I feel like I will always be alone, and that even if I manage a short relationship, they will leave me and I will be alone again. Can anybody help me? What can I do? I need help :(