Clearly it *wasn't* "that girl."
I spent three years with someone at one point. Thought we were the best thing since sliced bread, and that the sun shun out her ass. In hindsight, she was a lazy, shallow bitch who couldn't be bothered to do anything meaningful with her life.
Right now you can't possibly imagine there's anyone better, and that's perfectly natural. You're trying to adjust to how it feels without that person being such a huge part of your life. But someone more awesome *will* come along, and you'll remember this thread in a few years and think to yourself "Wow, did I really feel that way back then?"
And seriously, STAY THE FUCK AWAY FROM NEW RELATIONSHIPS FOR A WHILE! The best ones are ones that just "happen". If you start chasing tails, you're not the only one who'll end up getting hurt in the process.