League of Legends (LoL) Question: How do I stop throwing games?

I have a problem. I can't win games. I just can't. I'm going to give you an example game that I made up. The thing is EVERY game is like this one. I MUST be doing something wrong because I have lost 5 games in a row that I should have won. Ok here's how my average game looks. (I usually play ad carry.) So my laning phase is fairly good. For some reason I'm always ahead by at least 20 cs at 10 minutes and I often get a couple of early kills. I have the bot tower down faster every game (not boasting and you'll see why later) than the other team. The rest of my team is doing just as well. I've made sure that our team comp isn't one that just falls off late game and it isn't one that has power in the early game. We always have at least 2 tanks so our frontline is pretty good. Ok so we get the tower and my support and I go roaming and helping other lanes. We have a few drakes under our belts as well because bot is being constantly pushed. For every tower they take, we take 2. Things look good. We make a huge push and get an inhib. Before they respawn and we head to either dragon or baron. After securing that we recall glad that it was such an easy game. But here's where it all goes wrong (in EVERY game I've played for a while). We can't stay grouped. We can't maintain map pressure. I have no idea why it falls apart but it does. Every game. They have a massive comeback (I think its safe to assume we have easily a 5000g lead) and win. They group and we try to fight 4v5s because someone wasn't there when the fight started. They make a push and now its us on the defensive. While 4 people are taking our mid tier 3 someone is splitpushing top. If we send someone the pusher will kill them. If we send 2 mid will fall. It starts getting depressing that we threw such a perfect game. We usually just ff or fight hard and lose. Not only does this happen in ranked but in normal too (not that I care as much) I really want to stop losing and gain some lp for a change. Here's a game like this. Check them out and see if there's something I'm doing wrong.Thanks Game