Really... don't sweat it. You haven't actually lost anything, you will find that you have times when you feel you are on top of everything and completely untouchable, and a week later you may feel the way you described in your post. No one feels invincible 100% of the time. Life is never constant, and you can get that fleeting feeling out of nowhere for no certain reason sometimes, and it seems an especially common occurance right after a relationship has ended, no matter the reason for it.
Remain focused on yourself, and the psychology that began to transform you from AFC to master of your universe originally. Revisit some of the material/instructors that you felt an affinity with and provided those "Ah-ha" moments for you. It is also important to remember that this is just in your mind - you haven't lost anything, you are still a generation ahead of where you were when you started... and that these feelings are temporary. I assure you, even the gurus have them. Just another learning experience, and once you've gotten through it, you will be even better equipped to deal with in the future.
Good luck and happy hunting
