You're Soldier 76.
You've lost two rounds so far. Your teammates mention for the seventh time that the enemy Pharah is easily killing everyone, ask why you aren't focusing on attacking her, and one of them asks if he can play S76. You pretend not to hear them and silently refuse to switch. The third round begins. You focus on killing what's right in front of you, which is almost never Pharah since she's usually flying over your head. You lose the match.
What went wrong?
I didn't do anything wrong! Our Junkrat, Reinhardt, and Mercy should've been killing the enemy Pharah! It's their fault that we lost, not mine!
#1: You helped the enemy team.
#2: You didn't cooperate with your teammates.
#3: You treated Overwatch as if it's Call of Duty.
#4: You chose not to play your character effectively.
#5: You were too obsessed with playing a specific character.
#6: You entered competitive mode before you were ready.
Mistake #1: By choosing not to play your character the way they're meant to be played - to their full potential - your decision brought your team several steps closer to losing. Bad decisions like this result in your teammates dying far more often, enemies building their ults more quickly, you and your teammates building yours more slowly, and your team spending too much time respawning and walking back to the frontlines when you all should be fighting. All of this can easily be avoided if you just do the job your character is meant to do.
If you're going to make selfish decisions which bring your team closer to losing - which is no different than choosing to AFK/grief in the middle of a match on purpose, or choosing to do anything else that makes your team lose - you should consider being less selfish in the future, or quit playing competitive until such a time as you can learn how to either play the characters you choose, or at least be willing to switch off of them when a teammate is eager to use them to do the job that you can't.
Mistake #2: This is self-explanatory.
Mistake #3: You may be used to playing FPSs where the only tactics in the game are "See enemy, approach enemy, shoot enemy." Overwatch is not such a game. Many characters' playstyles revolve around doing more than just immediately attempting to kill every enemy they see. (This is also known as "tunnel visioning" your enemies, because you're focusing on nothing but them.) S76 is a perfect example of this. If you think "I'll only kill what's right in front of me at ground level!!" when there's an enemy Pharah over your head who's regularly getting double kills through team kills with well-placed rockets and ults, you'll never win consistently; you need to be aware of your surroundings and the enemy team's composition. If following this advice sounds difficult, consider waiting until you're several years older before playing competitive mode.
Mistake #4: This is self-explanatory.
Mistake #5: This is basically self-explanatory. If you're not going to fulfill a role that a character excels at, especially when choosing to do so will make the difference between winning and losing, then you should switch off of that character when your teammates ask if one of them, who is more skilled with that character than you are, can use that character. This might sound crazy, but your teammates aren't asking that for no reason: they're asking that because they want to win, and if they win, you win too. Don't forget that screwing over your teammates screws over yourself too. Always try to cooperate with them.
Mistake #6: In consideration of all of the points above, it's clear that you're not ready to play competitive mode. Play quickplay to improve, and don't return to competitive until you're willing to cooperate with teammates and can resist your impulsive, self-destructive, button-mashing urges.