1. That relationship that you wanted so badly to work just didn't. Because you learn that sometimes love and a whole lot of super-hot hookups (that may or may not have started with a post-11 p.m. text that read "U up?") isn't enough to keep a relationship together. He also has to, you know, live in the same city as you and want the same things you want.
2. You never lost five pounds to get that "bikini body" like you thought you would. Because not only have you moved onto stylish novelty one-pieces with cool cutouts and mesh insets, but also, you learn that there is no point in killing yourself to lose a little vanity weight when you're already a hot piece just as you are.
3. Ending a friendship with someone you always thought you'd be friends with. Because you learn that when people change, relationships change—and sometimes the person who changes is you. Your "friend" can ride off into the sunset on casual weekend cocaine binges and her never-ending need to feel superior to you and your accomplishments, and you'll ride into the sunset with a glass of rosé and a squad who actually cares about you.
4. Deciding you won't drink too much when you go out on a weeknight but—Oops!—you get wasted anyway and the next day at work is hell. Because you learn that balancing an adult professional life and personal pleasure takes effort that is actually worthwhile. You felt drunk all morning and hungover for the rest of the day and mentally hazy in an important meeting and you realized you never want to feel that way again.
5. Getting fired really early in your career. Because you realize that a place that fires you isn't one you want to work for. Besides, getting fired was the push you needed to leave and take another step toward your dream job.
6. Wearing something trendy because everyone else is wearing it, not because it makes you feel good. Because you realize you are your own woman and do not deserve a full day or night of discomfort in effort to copy some trend that makes sense for Kylie Jenner, who probably doesn't have to wait in line or ever fight for a cab after a night out. You learn your clothes should make you feel good and if you just want to be comfortable, so be it.
7. Blowing a whole paycheck on something essentially useless like jewelry or a purse. Because you came to realize that gold bracelet was actually a plane ticket.
8. Your partying interferes with your personal life. Because you learn that while partying can be the FUNNEST, you realize that it makes you feel really gross when you're too absorbed with your own personal Instagrammable funtimes to remember, like, your dad's birthday.
9. Not speaking up for yourself. Because you learn that the only way to get what you want is to ask for it. Besides, after you leave too many outings or meetings wishing you had told someone where they can shove it, you just get sick of it and stop holding back when people are assholes.
10. Not wearing sunscreen. Because sometimes you just need a terror burn from one ill-fated day in Boracay with too many margaritas and unnecessary dancing in a pool music played by an actual pool DJ to learn that no amount of partying can fend off a sunburn. Only waterproof/sweatproof/danceproof/everythingproof SPF can fend off a sunburn (and the resulting wrinkles and skin cancer) and you need to apply it early and often whenever you're around the sun.
11. Being too absorbed with your phone/Instagram to live your actual life. Because you only get one, and eventually you realize that if you died tomorrow, the last thing you would want to look at on your deathbed is your cell phone.
12. Staying in a bad relationship for too long. Because you learn very, very well what you should always avoid in future relationships. No, you'll never get all that time you spent fighting over the mattress you bought together back, but you'll have a near lifetime of not dating mattress-obstinate assholes ahead of you.
13. Obsessing over someone who isn't interested in you. Because again, you learn who—and what—not to waste your time on. If he only wants to date women who are fine with never meeting up on weekends (and probably also, like, never getting an orgasm from him) then he's obviously not for you or any other woman with a sense of self worth.
14. Comparing yourself to other people. Because you learn that what other people do actually has no effect on your life and comparing yourself to them is about as valuable as an empty Starbucks gift card.
15. Being too obsessed with how you look. Because you learn that you're actually at your most beautiful with minimal makeup and your hair unbrushed. Which not only saves you money but also enough time that you will stop being late to all your get-togethers.
16. Being too hard on yourself for the occasional indulgence. Because you learn that you are always your own best date. If you won't buy yourself an overpriced drink from a pool bar on vacation once a year or book yourself an impromptu deep tissue massage, who will?