There's a few things to keep in mind regarding this.
Fact 1:
League of Legends is, at its core, a team game. Regardless of how good you are, you won't win a 1v5, and if your teammates play badly enough, there may be nothing you can do to save the game. There are, in fact, some games that your teammates lose for you.
Fact 2:
Despite this, individual skill still plays a part. If you consistently play better than your counterpart on the other team, you'll have more power in teamfights, and be able to assist your team better overall. Although being a good player won't get you a 90% win ratio, if you're better than the people around you, you should be able to get something along the lines of a 60% win rate. In short, many people who feel they don't belong in bronze are often at least partially to blame for some of their losses.
Common problem 1:
There's many important skills that most bronze level players lack. The first one is map awareness. One of the easiest ways to die is by wandering around in the map when you have no idea where the enemy team is. Then all of a sudden three champions jump out of a bush, and kill you with little trouble. Many players will attribute it to bad luck (or worse, blame their allies/their champion).
The solution:
Fortunately, it's pretty simple to avoid this, with the invention known as the sight ward. Wards are a mere 75 gold (or 125 gold if you want the upgraded version), and give you vision of an area for three minutes. By placing these strategically at choke points, you'll know where the enemy team is. This lets you avoid their attempts to gank you, and lets your team set up ambushes for them. Keep in mind, a champion kill is (on average) 300 gold, with another 150 assist gold. That means if it takes four wards to avoid dying once, you still broke even. In practice, they'll save your life far more than that.
Common problem 2:
A lot of players simply don't know how to teamfight. There's five enemies pushing mid, and your team decides to engage despite two of your players being somewhere else. Thirty seconds later, all three of you are dead, and only one enemy died. Then the other two come back to try to stop the push, and they die as well. The enemy gets the ace, a couple towers, and perhaps the game.
The solution:
In the above scenario, you did not lose a teamfight. You lost a 3v5 and a 2v4, both of which are heavily in the enemy team's favor. Quite simply, the team with more chamipons has the advantage. Unless you're very ahead and can take a couple enemies out of the fight quickly, don't engage when you're at a disadvantage. If they jump on half your team and murder them without taking damage, don't throw your life away trying for revenge, just run! Above all, act as a team. If they blow all their abilities killing your tank, jumping into the 4v5 can potentially work. Having all four members of your team stay back under the turret is another option. However, if two members of your team go in and the other two retreat, you're just giving them free kills. Work together, or die alone.
Other notes:
Once you get a better feel for teamfighting and how to control the map, don't be afraid to try to take charge and suggest things to your team. However, make sure you remain polite when doing this. I don't care if you're 13-3 and your ADC is 1-6, if you're yelling at him for his poor play, he much less likely to listen to you. Try to control your frustration and offer constructive criticism, and perhaps go down and help out his lane. In short, if you can keep hurt feelings to a minimum, you have a much better chance of working as a team in fights.
TLDR:
Remember, individual skill in the laning phase is only one part of being a good player. If you have no map awareness and no teamfighting ability, you're going to die a lot in mid-late game, and probably remain stuck in bronze. If you can improve these aspects of your game (and encourage your teammates to do them as well), you'll find yourself dying less, getting more kills, and overall winning more games.
Hopefully this helped, and good luck on the fields of justice.
