How do you know when a 2 year relationship should be over?

I've had this pit at the bottom of my belly. I know that my boyfriend isn't the one, and I don't know what to do. We are both seniors and about to go our separate ways. We have been together for almost 2 years. I don't want to hurt him. I can't hurt him. We are so use to each other, and I'm so scared that I'll go crazy without him. I'm literally so confused, I can barely breath. Has anyone been in this situation before? A long term relationship that needs to come to an end, but you just don't know how to bare the thought of being alone?