First off is the quote "You are comparing your behind-the-scenes to everyone else's highlight reel." People bring up this quote a lot in regards to social media, but it actually applies to every interaction you have ever had with anyone else in your entire life.
People don't tell you about all their little failures and they also don't often tell you about their deepest fears, regrets, insecurities, and so on. People will be eager to tell you when they got an awesome new job, they will be not so eager to talk about how they've sent out 100 applications and no responses. And even when they do, it will be at a moment of their choosing, and framed in a way that makes them look not so bad.
Thus, you actually aren't comparing your life to anyone else's. You are comparing your life to what people choose to share about theirs.
There is also the rather mathematical impossibility of winning a lot of these comparisons, especially when you focus on people who have it better than you. Jonny is better than you at math, Jimmy is taller than you, and Jenny is better than you at gym class. Even if you are actually much better than average in all these respects, even among your peers, that's not what you see when you compare--you just see all the ways people are better than you. So it's just lose, lose, lose, everywhere you look. And even when you are the best around instead of feeling good you just stop caring about that and focus your attention on something else where you're not.
Another thing to consider is that each person is different. Everybody is so different that it is rather insane to even compare two people and say one is better than the other. That guy who's better than you at French? Maybe his mom watched a lot of french movies and so has he. That guy who beats everyone in basketball--he goes right home and practices, watches basketball, and he's been playing since before you were in kindergarten. Did you do those things? No? Would you trade your life and past for one filled with playing basketball? No? Then why are you feeling so bad that he is better at basketball than you?
I find it is still hard to stop these comparative thoughts from cropping up, but you just have to know they are useless and dismiss them. Don't chastise yourself when you have these thoughts, they are natural. Focus on your own goals and your own life and go after what you want. Take care of the people you care abut. Stand up for what you believe in. Try to leave the world a better place than when you found it. Chase your dreams. That's all that matters. Be happy there are other people around you with good qualities, or people who are better than you at things you want to be the best at. Would you want to have your current level of skill and be the best in the world? No.
It is true that some people will have advantages you do not have. You and the prettiest girl in the school both want to become actresses. It's not going to be as easy for you. You want to start your own business. It's not going to be as easy for you as it will be for the trust fund kid whose dad has money and connections in the industry you want to be in. You want to go to Harvard. It's not going to be as easy for you than it is for the kids whose parents are alumni, and paid for tutoring and violin lessons and sent him to study abroad over summer while you worked a shitty job just to save up for tuition. Life is not fair. This will be shoved in your face over and over again. You can do everything right and still fail where another person could have done everything wrong and succeeded. You could eat healthy and exercise and be diagnosed with brain cancer at 19 while a 110 year old drinks and smokes every day.
You must seek out the good in your life, and the good you can do, and focus on it. You are never gonna be anybody else but you. Play the hand you were dealt as best as you can. If that means you have to go to extremes to live the life you want, then go to those extremes. Many of the people you envy have gone to those extremes already without showing it. You can either spend your life whining to yourself you weren't born a supermodel billionaire pro athelete supergenius ruling monarch, or you can do what actually matters to you.