Why do we let things affect us so much?

Emotions seem to be out of our control. But they truly aren't. There is a reason behind every emotion you have, and you've brought it upon yourself. You are NEVER a victim to emotion. You've agreed with what you're going through, you have accepted it as truth, as regular, as you. But if you realize that everything is based off of your belief system, you can alter those systems to offer you better ground for decision making. Sometimes it feels like someone's words have physically wounded your heart -- the words contradicted everything you stand for. The physical pain is a direct result of you taking something personally, letting it "affect" you. But they're just words! Or it was just an action! Nothing touched you, nothing penetrated you ... but your thoughts are making you feel awful, aren't they? Depression is so common because we pile on all of this non-physical "disagreement" and let it build up until it's weighed us down so much we can't get ourselves to move! We're physically negatively altered from this unreal, transparent energy.

Thing is I don't know how to control this myself ... I've gotta alter all my current beliefs so that nothing affects me and I can stand on my own. Hard to do.