Enter your fifth decade grudge-free. Grudges can eat at you long after the original insult or slight has passed. In fact, if left to fester, they can actually intensify. They not only cause ulcers, they cause frown lines. You're past the mid-way point now, even though we insist on calling it middle age. Why waste your remaining years looking back at the roads not taken? Former spouses that were pure evil, childhoods straight out of "Mommy Dearest," friends who disappoint -- let it all go. Forgiveness is more than just the higher road. It makes you feel better, so allow yourself to forgive.
2) The need to win every argument.
Sometimes you must choose whether you want to be right or you want to be happy. You don't become a pushover just because you sometimes cede to another's view. You can compromise without being compromised.
3) Life deadlines.
When you are in your early 20s, you might have thought you'd be a millionaire by the time you hit 30. Since most of us learned that there really is no "right" time to marry, start a family, buy a house, or switch jobs, the message to carry into your 50s, 60s and 70s is that there really is no "right time" to stop work, travel to the places you want to go, dance naked in the rain somewhere. Do what you want to do and do it now -- because there really is that one big deadline that we all face.
4) The idea that you can eat whatever you want and not gain weight.
Sad reality check time: Your metabolism slows as you age. If you want to eat the way you did in your earlier years, you are going to have to really crank up the cardiovascular exercise. At the end of the day, it's about how many calories you burn. Also, enough already with crash diets, fad diets and diets de jour. What works is eating balanced foods and exercising. Not a magic cure in a pill bottle.
5) The belief that you can change other people.
While you may have figured out that the guy you married 20 years ago who took seven years to finish college and couldn't hold a job is probably never going to out-earn you, it's time to decide if you are OK with it once and for all. People don't really ever change; sometimes a few behavioral tweaks occur and that's enough. But if you want to see change, it's all you baby.
6) The expectation that others will save you.
Some people just seem to live under a dark cloud. They bounce from one jam to the next and always turn to others to provide the solution for them. Sometimes they want to borrow money, live in your guest house rent-free for awhile, ask you to donate miles so they can travel some place they want -- make that "need" -- to go. Whatever problem they have, they deposit it at your feet and allow you to feel good by fixing it for them.
We know someone who just launched a crowdfunding campaign so she can quit her day job and pursue her passion. Mind you she's not looking for investors with whom she will share profits if her project takes off. No, she wants people to give her money -- gifts, not loans. Now, feeding hungry kids, rescuing abandoned shelter pets, finding a cure for cancer -- we recognize them all as charitable causes worthy of the coins in our pocket. But quitting your day job to chase your dream while I still have to go to mine? Sorry, not feeling it.
By the time you hit 50, it's time to stand up on your own two feet. Take responsibility for yourself.

7) Thinking that doing it their way will make people like you.
Saying "no" is hard for people. We want to be liked, so we tend to err on the side of being agreeable. But you really can't please everyone all of the time and turning 50 is a great time to stop trying. Life is your party, after all. And speaking of parties, hats off to First Lady Michelle for doing her 50th birthday her way; if she'd invited us, we'd have no problem with what's become known as the "EBYC" -- eat before you come -- edict.
8) Speaking ill of people.
Your mother probably taught you that if you don't have something nice to say, don't say anything at all. It's time you listened to her. Mean-spirited gossip doesn't become you. You are a more attractive person if you don't engage in it. And as for those who attempt to lure you to the dark side, remember that as soon as you turn your back, it will be their knife you feel in it.
9) The old stuff on your resume.
Chances are, nothing you did 30 years ago bears any useful resemblance to the skills you need to land and succeed in a job today. Your resume should explain how you've spent the past 20 years and that's all. The other thing is, welcome to the world of age discrimination. Why let your resume date you? You should also probably lose any notion that a job is beneath you or that your age alone entitles you to respect or to command authority.
10) Your definition of success.
Success wears many faces. If the only face you see is linked to your job, something is wrong. Raising happy kids, being able to love unabashedly, having a life filled with people who smile when they see you coming -- isn't that success as well?
Arianna Huffington and Mika Brzezinski are taking The Third Metric on a 3-city tour: NY, DC & LA. Tickets are on sale now at thirdmetric.com.