Isn't that part of the problem though? It seems to be a catch-22: you need to have had a serious relationship or two to know that you are loveable, and you need know you're loveable to be in a serious relationship.At the end of the day, what makes me think that I'll be loved again is the fact that sometime before, someone loved me. If someone could love me once, someone can love me again.
If you've never been able to have a serious relationship, you need to go back to step 1: you, and work on that. If you've been in a serious relationship before but aren't, and would like to be, but no one seems to want to date you, you should do the same. Reassuring yourself that you can be loved in the future because at one point in the past you were loved is like saying, "Once upon a time there was a garden of lilies on this hill. Therefore, in the future, there can be a garden of lilies here again." It doesn't mean the garden of lilies will magically spring into being without you doing anything. With that in mind, if you have never been in a serious relationship, the only thing missing is the knowledge that what you want to happen has happened before (isn't impossible). While knowing love isn't impossible is reassuring, it is not going to make you any more currently love-able.