I lose games for my team

Hello, I could not find any topic about this (all I get is "I am good, they are bad" topics). So, basically I realised that I am often the reason, why my team loses. I know that I am a bad player, but I thought I would improve over time. My gameplay is focused on helping my team a lot, I buy wards and I try to clear enemy wards, I try to not steal any kills from my teammates, leash for them and block deadly skillshots. I tell my team when somebody is not in lane and I try to ping any danger, that has not been noticed by my team. I also usually let everyone take the role they want, so they can do better, all because when it comes down to positioning, plays, teamfights, dodging skillshots, farming, trades, laning or ganking, I simply suck hard. I either get crushed early and then enemy just snowballs out of control or I make mistakes in teamfights and I get caught often (usually when I try to place the wards to prevent somebody getting caught). Then it is just an easy game for my enemy. Sometimes my team can carry me, but on the other hand, when we lose, it is usually only my fault. Some people like to point it out in a rather impolite way, some are just dissapointed and I feel guilty then. The problem is that I feel like I do not improve anyhow, I read guides and watch professional gamers but it feels like if I hit my skill roof (and that is a rather low roof). I do not remeber any game when I carryied my team and I almost always do badly in any solo lane or jungle. I can sometimes do quite well in bot lane, as there is a other player to help me. Maybe it is my mindset as I often think, that my enemy must be better than me, therefore I tend to play more defensively in fear of getting killed (they can usually kill me anyway). If I played aggresive, I might have lost even harder as it takes longer to run to safety. But it is hard as my opponent can roam and kill my more valuable teammates, when I play under tower. If we lost any game because the enemy team was better, I would have been completly OK with that, but now it is just a lost game because I did horribly. Can you provide any tips?