Let go of high school, where it was easy to get more A's than anything else on your report cards and have friends and tangle with a job and maybe be on a team sport or two. You are not a teenager anymore and you are not living with your family. You are not allowed to look back on your days of so-called, blissful, unimportant glory and tell people you used to be a certain way, that you used to do it all and you're not sure what happened.
Don't complain about your mediocrity. If you're used to the good life, then it is going to hurt when you lose your chance at a promotion or that you don't understand the difference between grapes grown in Napa Valley or in Sonoma Valley. You can't be good at everything and if you hate it that much, your time spent fishing for compliments that you are a special little snowflake could probably be better spent doing something different to make sure that you can do some sort of preventative technique for next time. Unless you truly think that aliens abducted your brain, you should probably rethink your choices. Either work harder or figure out that, maybe, what you're doing isn't best and get out before it's too late.
Accept that, statistically, people will be better than you. It is impossible for everyone to be above average. Unless everyone in the world has the same level of achievement, or same level of perfection where we all get a Good Job sticker on our work no matter what, there is always going to be an average. And, for the most part and depending on the bell curve, there's usually going to be more than you will ever realize. For now, let go and let math tell the truth; that's one thing it's good at.
Stop comparing yourself to others. Stop going on Facebook and reading about people who "did so average it hurts" (completely true) and who go to California all the time and have a best friend and a boyfriend and go school in any other state but yours or have jobs with paid vacation time. Sometimes, mediocrity is okay and you have to accept that even if you don't have twenty-three friends, you do have one or two, and that you do have a job that pays for your life and you can afford things that you need. Because, even if there are people that are better than you, there are those who have less too.
Focus on what you can do. Don't think about all the times that you could have passed that test if you had just gotten one more question right. Stop thinking about the one time you did fail and the one time you did make a mistake. Think about all the other times that you've done something right. Think about that person you hugged or that ice cream sandwich you shared with a friend (but don't think about high school. That breaks the rules and, let's face it, high school doesn't count). No matter what you believe or what anyone else believes, you are here for a reason and can do something, no matter how average.
Don't let people drag you down even further than you already think you are. If someone is telling you that you are a failure, even in jest, don't let it slip by you. Don't let them continue to bring you down, saying that you're good for nothing, that you're dumb, that you're awful, that you suck, that you're not a good friend. After you don't invite your friend to something and they tell you that you're an awful person because of it, it allows them to slip into your heart and tear you apart from the inside out. While it's okay to be below average too, it's not okay to feel so low that you don't deserve to even be alive anymore. Even the lowest end of the bell curve goes on forever and doesn't hit zero: you shouldn't have to feel like one either.