My nephew's GF of 2 years came to me just last month DEVASTATED by exactly the same thing you have just experienced. They are both only 21 yrs. old. This had just come out of nowhere for her also, so not only are you having to deal with the break up, you ALSO have the mind-boggling CONFUSION about "what the **** just happened here??!! I truly believe it is soooo much harder when you just CANNOT for the life of you put your finger on ANY reason! I do NOT think you are naive in any way...this DOES sometimes happen FOR NO REASON at all and it is made worse by people thinking that you must have just been blind to the "problems" that there MUST have been in order for 1 person to end things so suddenly. You are not blind and, most likely, have truly not DONE anything to warrant this sudden demise of such a long-time relationship.
All I could tell Kelly was this:
DO NOT CALL HIM FOR ANY REASON!!!!!!!! Sit on your hands, call me, come over here even if it is 4 in the morning, whatever it takes! Here's why: if he told you there is NO reason, that he's "just confused" or he "needs some time", etc. you will get NOWHERE with him by calling him, trying to see him, etc. You MUST make HIM wonder about YOU!!! Everytime you call him, or go over to try to see him, etc, he'll know EXACTLY where YOU stand, how you feel, etc. He WILL HAVE NO REASON TO WORRY! If, on the other hand, you DO NOT behave as he is EXPECTING you to behave, he WILL have reason to WORRY!!! "Hmmm, what's going on here"? "Why isn't she trying to get me to come back"?, etc. etc...
It took EXACTLY 19 DAYS for my nephew to call Kelly!! She DID NOT call him after the very first day that he broke up with her. It was VERY, VERY difficult for her and she was here or on the phone with me more times than I can count (unbeknownst to my nephew, of course!), started going out with her friends, ran into him twice during this time while at the same bar and she and her friend LEFT to go somewhere else!!! I was sooo PROUD of her, as that was the most difficult thing she had to do!! BUT, THAT is EXACTLY what brought HIM to HER door the VERY NEXT DAY!!!!!!
Now, there is obviously no guarantee that this will work in all cases, but it is ALMOST a sure thing! If it does NOT work, then at the very least, you will NEVER have to feel ashamed of anthing you tried to do or anything you said, and that is ALSO good for YOUR SOUL!!
Keep your chin up, dive in the shower, do your hair, throw on a bit of makeup and HEAD TO THE BEACH TODAY!!!!
Good luck, and remember, the DIS is ALWAYS OPEN, even at 4am!!