That feeling has been very real for me. There's a lot going into that emotion, and I think none of it would sound new to the bulk of our cohorts here. You spend 16 years striving to earn a degree (in my household, anyway, 1st grade was a service done to the years following). Feels weird to see it transform from a moral inheritance from my parents to an 8'x11' piece of paper - it goes from your life's work to something passive and dull so quickly. I'm getting used to the feeling, though. I doubt it ever goes away, seems like one of those things you just learn how to tolerate. (sparkle sparkle)Out of the blue, beyond any cause you can trace, you'll suddenly realize things are not how you perceived them to be at all. For some reason, you will no longer be the person you believed you once were. You'll detect slow and subtle shifts going on all around you, more importantly shifts in you. Worse, you'll realize it's always been shifting, like a shimmer of sorts, a vast shimmer, only dark like a room. But you won't understand why or how.
It is more about the journey than the destination and all those you meet along the way. The degree / promotion / new job / [achievement] you achieve marks an end to a chapter but doesn't take away your story on how you got there. For each new chapter, accept that it may not end the way you intend. Just take some time to enjoy the road along the way. Life sometimes gives you shit (and a lot of it at times) but life will pleasantly surprise you if you keep your heart and mind open. Change is inevitable so be the change you wish to be and accept change you cannot control. Shimmering can be a good time to reflect. At least that's how I have come to terms with that feeling. I apologize if this comes off like a bad self help book, but this is something I have though a lot about when reflecting on my own life.
Sometimes the advice in bad self help books makes its way onto a page because it's good. For the record, you didn't come off that way. Let's compare my life to a car - right now, the deceleration as the transition kicks me up a gear is discomforting. I'll start cruising again soon, though! In the meantime, I'll use this shimmer for self-reflection as you suggest. Thanks for the wise words :)