You're protecting teammates with your shield. Suddenly you charge far away from your allies in an attempt to kill an enemy. You fail to kill anyone, swing your hammer a few times at enemies who saw you coming from a mile away, and die within seconds. You left your teammates to die in a 5v6 in which they don't have a shield, but the enemy team does. This repeats several times and the enemy team eventually wins.
What went wrong?
I didn't do anything wrong! Killing yourself is how you win matches! It's my teammates' 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 entered competitive mode before you were ready.
Mistake #1: By abandoning your team and getting yourself killed in a stupid, easily avoidable way, your decision brought your team several steps closer to losing. They're far more likely to be outnumbered - and thus wiped out - in their next fight, making all of you waste anywhere from half a minute to several minutes regrouping, depending on how focused everyone is. Bad decisions like this also make all of you waste precious time that could've been spent pushing in together and building your ults, and feeds your enemies' ults by getting your entire team killed over your impulsive decision.
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 resist the obsessive-compulsive urge to charge into groups of 3 or more enemies who will inevitably kill you within seconds.
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.) Reinhardt is a perfect example of this. You can't just think "Kill, kill, KILL!!" as the most defensive character in the game and expect to win consistently; you need to use your shield and be aware of your teammates' needs. 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: 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.