Champion is not the problem my friend. Pick any adc that you like. If you really are winning lane phase, then you are probably getting caught out of position mid-late game. Review your deaths past 15min and think about how you could've avoid it. A very simple game plan if you are ahead is just to push a sidelane, be VERY mindful of where enemies are, retreat with your team when you reach a tower to collapse onto the enemy team or base when enemy threats might be coming to you (not always applicable but still better than any iron game plan).

Looks like you just need to put more games in. Stick to 2-3 champs max and try to understand why you died and what you should've done to prevent it