You're the little boy who got a puppy for his birthday and loved it so much that he just kept squeezing the puppy tighter and tighter because he didn't want the puppy to escape until he smothered the puppy to death. The thing with love is that if you hold it overly tight not only you will eventually smother it but you also prevent it from having any room to grow and expand.
You have to recognize that you are basically the only guy she dated as an adult, and either she was gonna meet someone better and bail on you after committing (thus hurting you) or part of her would always wonder if she she could have met someone better (thus resenting you by forcing the issue when she wasn't ready). By making the ultimatum you gave the the options of hurting you now or hurting you later while resenting you for the rest of your relationship by forcing the issue.
What you should have done is (and I will admit that this is may seem completely idiotic at first glance) told her that she needed to spend this summer dating other people. That in order to prove to both of you that you are mature enough to be in a committed relationship with each other, she needs to sample what else is out their. Once you're back after the summer, the two of you can talk about where your relationship is going to go. Basically you would have been giving your permission for her to do what she wanted to do anyway. Make it clear that you want her the to do this and that you reserve the option of seeing other people also.
Make it clear i want her? And I can't still say that?

(your last paragraph)
And the crazy thing about all of this is, she was the one that always wanted to hangout, always wanted to be together, etc. at about 6 or 7 months together I told her that it was nothing personal, but that there are times I need my "me" time; time for myself and just myself. I was the one who felt a little overwhelmed and a little bit like, "whoa, this is getting serious and I love her, but I'm not ready to be together virtually every day." Things improved for us, until a month ago when, "like a switch," things just flipped and she felt a little overwhelmed and she's afraid of settling down now.
I have always made sure never to hinder her plans with her friends, impede on the rest of her life, etc, especially when I recognized that she was sort of doing so for me. I haven't been perfect on this, but I believe I've done a pretty good job. Part of me thinks that she got so into this and our relationship that she's almost burnt out, and while she loves everything I do for her, and our companionship, the burning desire to constantly be together is lacking. That's the "missing spark," that's why she's afraid of settling down; because she wants the feeling of always wanting to be together. It's probably a good thing for her, to realize that she can't depend on that feeling with me, or anyone, for that matter. (Also good for me to realize and see this first hand)
While I'm attempting to move forward, I firmly believe at some point our paths will cross again. What happens then? Only God knows. I do know, however, that it has become apparent to me things will never be the same, and we won't be together in the foreseeable future. Things will either be better someday, or worse someday, both of which will ultimately be good for each of us.
Above all, I just hope she's happy with whatever path God gives her.