Fact is, if you can't give commitment (whatever that was to her, marriage? living together?) to someone, they can't mean that much to you.
You really need to ask yourself – am I upset because "she has gone" or am I upset because "she is gone and that leaves me on my own"?
Some relationships look great on paper/feel great at the time, but if you both have differing ideas regarding long term plans (or lack of them) then it's gonna start falling apart.
You can't always tell when someone is the right person, but you do know when they are not. Guess she realised that waiting around for someone who couldn't give her what she needed meant they weren't the right one.
If you honestly want it to work with her, you're gonna have to put some serious effort in right now, take the plunge and get it sorted. But if the thought of that makes you feel a little empty inside then it's probably better she's gone.
Hope things work out for the best for you both. 🙂