A teammate asks if you or Junkrat can switch to Soldier 76. You quickly say "Yeah, fucking Junkrat needs to switch. He isn't doing SHIT." You have no medals. JR has 4 gold medals and only 5 minutes of play time as S76, so he says he should keep playing a hero that's clearly working in this case. You scream that you'll go AFK if JR doesn't switch; he's the one who needs to, not you, because he's not even a real DPS hero. JR complies. You all lose. Once the next match begins, you pick Reaper again as fast as you can.
What went wrong?
I didn't do anything wrong! Everyone needs to switch except me! It's my teammates' fault that we lost, not mine!
#1: You destroyed any chance your team had of winning with your toxic behavior.
#2: You helped the enemy team win.
#3: You didn't cooperate with your teammates.
#4: You were too obsessed with playing a specific character.
#5: You overzealously adhered to popular opinion about the current meta.
#6: You asked others to do something that you were able, but unwilling, to do yourself: switching heroes.
#7: You entered competitive mode before you were ready.
Mistake #1: Losing streaks aren't fun, and they can make people much more hostile than they usually are. But you know what's even worse? Making your losing streak last even longer by acting like a douchebag to everyone around you, including the people who are on the same team as you and want you to win. You know, because they're on the same team that you're on. Try not to forget that. Insulting your teammates is a good way to make them stop attacking the objective so they can respond to whatever rude thing you just said to them. You've probably even seen people quit playing in the middle of matches with you because of how you treated them.
You have the power to stop shooting yourself in the foot. You're responsible for your own actions and, speaking of which, you're not responsible for other peoples' actions. Just because you're upset that you're playing poorly doesn't mean that you get to project your own failures onto other people ("Ugh, you all suck! But I don't!") and claim other peoples' successes as your own ("I'm the only one who's doing well, even though I don't have any gold or silver medals!") If your ego is so fragile that you unironically feel the need to lie about these things in front of other people, consider seeking psychological help or waiting until you've grown up several more years before playing this game again, and I don't just mean competitive mode. People like that aren't even welcome in quickplay, practice, etc.
Mistake #2: This is self-explanatory; see all the points mentioned above. Driving your teammates to stop playing so they can reply to you, driving them to quit playing altogether in the middle of matches, driving your teammates to mute you so they can't hear the drivel you're spewing or your callouts, and tilting your teammates and yourself all contribute to helping your enemies win.
Mistake #3: This is self-explanatory.
Mistake #4: This is self-explanatory.
Mistake #5: Although characters are sometimes listed under the only category that fits them - for example, Mercy is clearly not a DPS, defensive, or tank character; she's only effective as a support hero - this isn't always the case. Take Soldier 76 for example: he clearly works well on both offense and defense, even though - *gasp!* - he's not listed as a defensive character. Shocking, I know. Just because a character catches you by surprise because you don't know anything about them - such as Junkrat, how insanely high his damage output can be, and how good he is at contesting points when played by someone who knows how to use him - doesn't mean that you should immediately dismiss people who play him, or any other character who's listed as a defensive one, on offense. Winston's another fantastic example of this. In short: if someone is objectively doing better than you, don't bitch about them. If that advice sounds too difficult for you to follow, consider uninstalling Overwatch.
Mistake #6: Imagine placing your hand on a burning stove, complaining about how much it hurts, and refusing to take your hand off of the stove. You then take this stupidity further by demanding that someone walk over and pull your hand off the stove for you. This is how you come across when you say "you need to switch!" several times per match while refusing to do so yourself. "But I need to keep playing this hero!" I can hear some people saying. "If I switch off of DPS to give our team two healers, then we'll only have one DPS!" Here's a revelation for you: perhaps your teammates aren't switching because they can't effectively play the class that you're asking them to switch to, but they can and will fill your role for you after you switch. Shockingly, they might even do a better job in your role than you're doing.
For example, say your team has 3 tanks, 2 DPS, and 1 healer, and you want a 2-2-2 team. You're DPS and you reluctantly switch to playing a healer to give your team 2 healers, but only 1 DPS because you switched. One of your teammates then switches from a tank to a DPS class, giving you the 2-2-2 balance you wanted, because they were willing to switch to your class (DPS), but if they had switched to the class that you had demanded of them (Healer), they knew they would do very poorly in that role, and so refused to switch since it would hurt the team's chances of winning. This ties in with one of the points mentioned above in an earlier mistake: you can't control other peoples' actions, but you can always control your own. If you want to get something done, do it yourself instead of crying for other people to do it for you.
Mistake #7: 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, until you're willing to treat teammates like teammates instead of enemies, until you're no longer willing to lie about yourself and other people due to your insecurities, and until you can play more than one character.