I think whenever we approach a big birthday we do a life check. I know I did one when I hit 30 also. Different life experiences but I still did one. For me, I found what was still making me happy and vowed to do more of it. In my case it was costuming so I upped the number of events I attended. Maybe you can do something similar. And if party girl is still on your list then make those changes you wrote about, I.e. Curl your hair, and not just wear sweats.
But if it's not, don't feel bad. We all change. Embrace the new you and fight to keep what of the old you, you want to keep.