My undergrad graduation was the last one I actually went to--the last time I walked across a stage to claim a diploma. (I was already back living in Rockford during my MA one and I skipped the MFA one and only went to a reading the day before with other graduates.) The undergrad one I remember only as a sticky, rainy day, where the ceremony was moved into the cramped gym and my parents had to stand behind the stage to see. I had bad hair and bad clothes under my gross polyester gown and really wasn't feeling like I accomplished all that much in the previous 4 years beyond the rote routine of classes and play rehearsals and just lots of reading. My mind was already set toward grad school and leaving as soon as possible. Within the month, I had moved into my studio in Lincoln Park and embarked on that particular version of adulthood for a couple years. The real plunge of adulthood surely happened a couple years later when I actually had to find a job (a real, full-time, benefits sort of thing.) Of course, I was in school again a few years later, but that was different, since I was already working full-time during.
But it's harder than I imagined it to be. But also better. The bonus is it might surprise you. The bad thing is it might knock you on your ass. You never know which is coming. I think the best thing about college was that semester's were bit sized and then you were finished. Could breathe for just a second before plunging in again next term. . I's dotted, T's crossed. Papers and projects turned in. Post-graduation life is missing these breaks, perhaps when you most need them. All your concerns that you had as a college student seem tiny when it comes to adult-type stuff. The endless hoops of grades and papers and exams. Their importance.
I guess the best advice: You will probably be nowhere near where you thought you'd be in 20 years. But then you will probably be exactly where you should be.