Try not to make the common low elo mistakes I finally figured out I was making last season. I finally realized I was making these mistakes after playing normals for most of last season looking to improve my "carry losing teams" skill. At this point, I could usually win my lane, but I was still losing about 40-50% of my games (sounds similar to what you described). After taking the good part of last season to really make an attempt to learn to carry teammates who lost lane, I call these patterns I noticed in my losing games the 7 deadly sins of lost games (listed below in no particular order):
**1. Defaulting to following your team around for ARAM midlane after laning phase.
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-This is probably one of the biggest things that comes to my mind when I remember things that held me back from winning games. This is the main reason that in season 5 and 6 I had 45-50% winrates and was hard stuck in low silver. The reason that doing this is so dangerous is because by always following your team and grouping mid to butt heads over and over you turn the game into a coin flip. It becomes which team has someone get blown up/caught out first and throw the fight or which team's carry gets focused last. Not a reliable method to carry at all in low elo; I know this now from experience. If I see my team still doing this when we're obviously losing teamfights, I'll usually splitpush or look to catch someone in the jungle instead.
Remember: Many team comps wont win through 5v5 fights alone, you may have to try other methods such as sieging, baiting objectives to catch someone, or split pushing depending on your team's composition and strengths.
2. Following a soon-to-be-dead ally into a losing fight.
-Also a big factor in losing games at low elo. I can't count the number of times I either participated in, or saw this happen in bronze-silver. If you see a lone ally with half health fighting 3 enemies who have mostly full health, do not be the "hero" who goes in to help and just ends up giving them a 2 for 1. Ping your teammates who try to help away from them, unless you have the clear advantage. It may seem cold to just leave them for dead, but it helps prevent even more deaths. Have personally seen numerous games thrown exactly this way at the 30+ minute mark, which leads to a pretty lame conclusion to a stressful and hard fought game.
3. Letting toxicity get to you.
-This one is fairly simple, don't let toxic teammates bring you down. Save yourself the time and headache by doing /mute all at the beginning and communicate via pings.
4. Chasing kills far beyond what you should.
-Happens mostly in bronze from what I recall, but chase enemies a reasonable amount (ex: until they burn flash, all your abilities are on cool down, or you risk dying by chasing them further) then move on if they escape you. Chasing a low health enemy for another 20s is a massive waste of time that just squanders any lead you may have and lets them get back in the game. Not to mention they could potentially bait you and your team into an ace if you chase long enough. Take/threaten an objective instead while they base.
5. Favoring kills over CS/objectives.
-Another big problem at low elo, CS/objectives should always be prioritized over kills. Don't get me wrong, kills are definitely a good thing, but far too often I have seen people drag out (and then lose) games by favoring kills, when taking objectives while the other team was dead or lacking numbers probably could have ended the game 5 mins ago. Have also made several comebacks from behind by focusing on CSing and objectives while the enemy team tries to pad their KDA and boast in all chat.
6. Forgetting to defend base/not pushing out lanes before fighting for objectives.
-This point is fairly important, but not really mentioned in most "escaping low elo" guides I see. The point behind pushing out lanes before grouping to take objectives is that if you lose the fight, the waves pushed to your enemy buys you time to defend while your team respawns. It's a play-from-behind tactic that can prolong the game long enough for late game team comps to become relevant. However, when doing this remember to ward up the jungle or river and constantly watch the minimap so you don't get caught while pushing out lanes. Forgetting to push lanes before an objective has sometimes lead to a near ace (3-4 dead on our team) near baron/dragon with a wave already pushing into our base, and the other team just waltzes in and wins basically for free.
7. Getting tilted and continuing to play ranked.
-Another simple point, but if you're tilted from last game just take a break. Emotions from last game can cloud your judgement and make you more likely to make a bad decision you would have caught had you not been tilted. I personally quit playing ranked for the day once I lose 3 ranked games (they don't have to be in a row).
Best of luck out there in ranked!