Hey there! High Platinum support here - take what I am going to say with a grain of salt.
1.) The very first thing that I notice is your death rate. Frankly, you are dying way to much. The first critical piece of advice I have for you is to die less. I know that might sound unhelpful, but seriously - I want you to shoot for 5 or fewer deaths per game. Take a minute and look at my match history, see what my average deaths per game are. Compare that to your own.
2.) Holy crap do you play a ton of champions in a lot of roles. While it's really good to be diverse, if you're looking to straight up improve and climb? Try focusing on only 1 role, and 1 or 2 champions. Pick your favorite/best and play ONLY them - also take some time to make sure they are in a healthy spot in the current meta. It's best to focus on a single role. Right now bot is the hotness, but Every role can carry very well if you know what you're doing.
3.) Dont take losses hard - even the best of the VERY best only rack up to a 70% winrate over the long haul - realize you will lose, and instead, focus on improving specific things during your games. For now? Be very careful about dying.
4.) Vision is more important than anyone thinks - this is doubly true if your playing below a platinum level. Buy more wards. Develop your play to be more cautious. Not giving up free gold gives you the best chances at a fair fight come late game. The reason why im so critical about your deaths is I notice another big trend:
5.) You don't have problems when your ahead - If you do well, you tend to do REALLY well. You run into problems when you fall behind. This suggests to me that your not paying attention to WHO is fed in the game. So start watching all 9 players in every game you play. You need to know who is strong, and who is weak on BOTH teams.
6.) Avoid playing champions that fall off come late game. You want to be able to CARRY late game, or have the potential to do so come late game. In Silver and Gold, many players do not understand fundamentals like pushing and lane pressure. So games tend to drag, even if they say 'league is shorter than ever' ... Because so many players don't understand how to END a game come early mid or mid game, late game champions are just better. As you climb up the ranks, this changes a fair bit. Right about Plat 1 / Dia 5 do you start seeing players really getting a solid understanding of how to end games early.
7.) Last piece of advice: Don't back too often, and be sure that the enemy doesnt get free damage on your tower when you do. I see people even @ plat levels of play do this often, and give up over half a turrets health.
Without watching you play, there isn't much else I can offer - Trading and Lane pressure are something that just comes with playing tons of games. So goodluck :D hope the advice helps.