There are some things that probably constitute failure, and none of them are the things we use to judge, measure and quantify our lives on a day-to-day basis. You've failed yourself when you've given up. You've failed the human race when you've maliciously hurt another. But all the other things? The numbers we use to keep ourselves in check? Weight and money and status and lines on résumés and notches on bedposts? Not doing those things perfectly isn't failure. It's just life.
One of the things you'll try to measure yourself by will be how often you can get your reality to appear the way you used to assume it would be. You will rarely succeed at this.
But you still haven't failed.
Even when nothing turns out the way you hope. Even if you don't save as much money as you promise you will, or think you've found "the one" five times before you realize that everybody you date is "the one" until they aren't, even if you overspend or over-eat or give into your indulgences or can't get that one mildly disturbing thought out of your head or think you're crazy or are (kind of justifiably) hated by someone or have to move back in with your parents or leave the city you just moved to or move twice more after that, even if you don't have a great credit score or weigh more than you did in high school or aren't living the life you dreamed you would be when you were 17...
You still haven't failed.
These things are not failure. These things are the process. These things are art. These things are healing. These things are the whole story. These things are annoying and nagging and anxiety-inducing and you may never be past them. One host of problems will replace another, over and over again.
You may not ever be out of debt. You may pay off the car you got when you were 20 and then take on a mortgage. You may pay off your student loans and then take some on for your children. You may find "the one" and even marry them and then still lose them one day. You may lose the weight then gain more as your body ages and sinks and grows with time. You may land the plum job and still worry you're not good enough to do it. You may get everything you think will fix your problems now, and then still worry about whether or not you'll get the things that will fix your problems tomorrow.
The point of all this, of course, is that ultimately, it's not "how much you have failed" but "how you coped with failure."
How did you learn to love your body, even when it was rounder and thicker than before? How long did it take for you to ask yourself: "Why am I overspending? What drives me more than my financial future? My clothes? My ego?" How much grace and gratitude did you have as you packed your bags and bowed your head to a city or a relationship and said: "thank you for teaching me that." How many weird thoughts passed through your head before you began to say: "ah, I see you, not today, thanks."
The place you're trying to get to isn't one where you don't make "mistakes." "Failure" is only something different than you thought would happen. The only things that happen are the ones we never think will. If you don't understand this, you will always "fail." But there is an alternative, and it's the stuff real success is made of. It's not seeking comfort, and it's not settling. It's stepping into every moment, every day, whatever it brings, and saying: "alright, this is what we've got today, now what are we going to do?" People who live happy and thoroughly fulfilling lives don't magically receive the best, they make the best.
You've failed the day you've stopped trying. In the words of my good friend, Johanna di Silentio: "Relax. You will become an adult. You will figure out your career. You will find someone who loves you. You have a whole lifetime; time takes time. The only way to fail at life is to abstain."