Why cant I improve my skills

I have trouble playing against assassins in mid lane, I sometimes dodge {{champion:53}} {{champion:412}} {{champion:111}} {{champion:142}} {{champion:19}} {{champion:25}} But the moment I fail to its a guaranteed death and its frustrating. I try to think of what items could be useful for the champions I play which are {{champion:267}} {{champion:45}} {{champion:29}} {{champion:136}} {{champion:16}} {{champion:50}} {{champion:106}} but more often than not I end up lacking damage, not being able to protect other teammates or taking too much damage because I have a hard time choosing to build tanky or do damage. I forget to ward a lot, and other times I feel like my reactions are too slow. I stay away ranked because I don't view myself good enough to really do much. Is there anything that can really help me? Also, a tactic or skill I could develop to deal with champions like {{champion:238}} {{champion:105}} {{champion:55}} {{champion:268}} {{champion:17}} {{champion:254}} {{champion:91}} Would help, I want to be able to not be entirely useless when they are around.
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