Whatever the reason for breaking up, when a relationship of that length ends it's absolutely devestating. Not so long ago, I was in the same kind of situation, although it was actually me that ended it (it's hard to explain why, but I really felt like we were arguing none stop and I just really needed a break and I kind of thought it would only be temporary, then he went and got someone else a week later, after three years...). But anyway, even though I had thought we weren't getting along, as soon as it was actually over I wanted him back, I cried non-stop and was a complete mess for awhile (like self-harm etc, seriously was a mess), I felt like there was no point in life, I kept thinking about him. Like you, it seemed like he had kind of been my whole life, I was incredibly lonely.
I also wished it would just get better right then, like when people said give it time all I could think was that I couldn't deal with more weeks and months feeling how I did. I wish I could say there is a quick fix, but there isn't unfortunately. Yes, you will be lonely, you'll feel empty and just really down, but it does get better eventually. I'm so sorry you're going through this, but I really do think you'll be stronger at the end of it. In a few months you'll probably feel like I am now, yes I miss him, and I still feel sad and lonely, but I don't cry all the time, I don't feel like there's no point in life. Hang in there, it's the only thing do to.