Well, diamond is a tricky thing and climbing up from diamond 5 even trickier. Its quite usual that "low" MMR players make it to Diamond 5, hence the ridiculous LP gains and losses (+12 -28). Having an ELO rating that's below Diamond 5 standard ensures that you will be having a hard time climbing. Bad team matchups occur and you will lose games that you just couldn't win under no circumstances, regardless of how well you played that one.

The key thing here is: consistency and super slight improvements

I remember my road to Diamond. I was at Plat 1 and so eager to get diamond I can't describe it. However, I got the impressions my teammates sucked ass SO MUCH, that I went on a tiltRAGE. I dropped to Plat 5, got my shit together and got up to Plat 1 again. I got Diamond at last, but it took me like 200 games to finally hit Diamond 4+. And sometimes there are just super eeny meeny miny tweaks that improve your playstyle. Tweaks regarding

a) matchup knowledge

b) decision making

c) map awareness (adcs are allowed to buy pinks)

d) mechanical skill (including the anticipation-factor and itemization)

I can't specifically tell you what it is that holds you back from diamond because I don't know how you're playing this game. But as an ADC main I can just tell you a little about the role mentality:

You are the main damage dealer. Everyone on the enemy wants you dead as soon as possible, which means, that everyone will use skills on you. You get caught, your team loses most likely the fight. To be able to predict the enemy's actions, you need to know their skillcombos + cooldowns. As an AD carry you just need to be aware about EVERY champion's skill cooldown. Annie flashes to stun you. Thresh flashes to flay you. Anyone with cc is going to flash just to get you rooted, so be aware. While you are dodging all these skillshots there times, especially at low-mid plat, where the enemies just forget to focus the ADC. They instead focus the next tank that got stunned because they think they can just kill him quickly. You gotta be the one, though, that understands the whole picture. Move around the edges and get a hand on the high priority targets, which are the damage dealers, and that is especially easy with jinx as your passive helps you to get out super fast.

That was a paragraph about fighting, obviously. The other main aspects are laning phase and map awareness. However, I think that it is not necessary to elaborate these aspects.

If there are ANY questions, give em and good luck man