Your existence is meaningless because you think it so. Just you, no outside cruel forces, it's all you. You know this but think otherwise. The difference between thinking and knowing? You know how to breathe without thinking about it, yet when you apply thought to breathing you can alter it's knowing, natural rhythm. You also know your existence has meaning but when you apply thought to the contrary it creates dissonance which manifests as depression. If you truly KNEW your existence is meaningless you wouldn't be depressed, you'd revel in it and experience joy every time you THOUGHT about the sheer meaninglessness of it all, knowing and thinking would be all synced up. This is how I know your existence has potential purpose. It has to suck having someone so easily see meaning in your existence when you can't see it for yourself and its the thing you want most of all.