Rejection is a huge issue for most people, but here's the good news, it isn't just you - it's meant to be distressing, humans are programmed to feel it that way. Research indicates that it is one of those protection reflexes over which we have no real control, helping us to learn from our (very painful) mistake and making us less likely to make it again (not that same one, anyway!). So why not stop beating yourself up for reacting "badly", being "pathetic", not being stronger?
We are social beings, we want to belong, be part of something, interact, share, but often we'll avoid opportunities to become involved in relationships with others because they necessarily imply the possibility of rejection. Which, as we've already established, hurts. The thing is, that unless we open up to that risk, we can never form deep, meaningful relationships with anybody.
One further complication is the perceived slight or brush-off. Was it meant/inadvertent/a joke? Are you reading too much into what was said/done? Is your interpretation correct, is that what actually happened? Perhaps a more appropriate course of action here would be to look closer to home, inside yourself, reflect on what it is that is upsetting you so much, because it may not be rejection as such. Then, once you can separate what is, in reality, your rubbish from what is theirs, you are more likely to spot true rejection; and true rejection is almost inevitable if you want to be involved with others in a worthwhile way. Learn to deal with it and you can allow yourself to be vulnerable, which is where genuine joy and happiness can be found. I think it's worth it.