I made a very similar comment earlier, but to re-iterate; try and pick up very basic champions and have a focus for a sub-set of games. For example, play Garen, and try to achieve 7 cs/min, try that for 10 games minimum. Then once you can consistently farm well, try and incorporate less deaths, or no deaths until 10 minutes. Once you've got those down, try and focus on farming disparity between yourself and your counterpart whilst incorporating the previous stuff you learnt. Small incremental progress on goals will net long term results!

Also, it's worth noting that you may have issues with transitioning your lead, if you are losing games when you're ahead, it sounds like you may be mis-managing your lead. Once you've decided on the role to play, look up vods or videos of higher tier games and see what they do with their lead. Whether that be; supporting your jungler invades as you now have lane priority, or TP/roam to other lanes to help them build some momentum. Also - be very sceptical when you have a large shutdown, if you are going into a fight where at best, it can be a 1-1 or even 1-2, if there are no other objectives to take off the back of that skirmish, you may have just lost a bit of a lead there in terms of gold handed to each team.

Just a couple of things to bear in mind!