Being overconfident could end in disappointment in many ways. For one, many people may not want to spend time with you, because you might brag or talk too much about yourself, and you'll end up losing friends. You could even lose connections with family. It could also result in disappointment because if you are too overconfident, and you still fail something, it would probably hurt your own feeling really bad because you were so sure you could complete something. Then, you would most likely end up with no confidence at all. (There are many reasons being overconfident isn't always good, but I hope these two were useful. -pastasantana on instagram) :)