So, everyone seems to be acknowledging the fact that your mind isn't as sharp as it was (maybe I missed a comment).
Thing is, it's simple. No, don't acknowledge it. Sure, there are hardened synapses, but to a large extent that's just an excuse for people to feel sorry for themselves. O_O
So basically, just get over the fact that you think your brain's not an sharp as it was. It's a load of crap. It's still all there, and you need to just overcome to emotional inhibitions that are getting in the way of you using it.
University is a pretty safe environment for exploring --as it should be; that's its purpose if nothing else. As a result, you're pretty well emotionally oblivious to the "what-if fears" of putting everything --all your mind and all your passion- into what you do. Perfect.
When you depart, a profoundly unseen transition takes place; you don't have that environment anymore. So you continue with the momentum you had for a while --couple months, 10 years- and then it's gone. You're learned a new environment that pulls into your mind before everything else a whole mess of things; the clichés of "only what's good enough" and "dreams are just dreams."
And then there's the "age 27 factor" as I've coined from the film Proof. (actually, go it is. It's appropriate.) :) Look back over old work and you think you realize that you'll never be able to do something that great again. Again, load of crap. Honestly, for myself, I'm typically looking back over old stuff and, while realizing the significant merit they held at the time and to the process of moving to where I am now, they're working in a highly limited fashion. What consistently happens is I let their singular weight become a symbol, a pristine concept for the knowledge I gained and I hold them so much more highly that what they actually are. On a personal level, that's fine. But to compare that symbol to actual work, no. Don't do it.
So here's my final thoughts on what it boils down to: Get over it. Your brain is still all there unless you've found little bits on your pillow. It's only a simple emotional inhibition that's preventing you from actually using your brain. Don't blame it on anything else. And finally, your old work wasn't that great. (ok, so maybe it was, but it's not nearly as great as you're holding it.)
That's it. :)