This could be a variety of issues. Most of which is likely the following:
1) You get carried away. You are mostly playing twitch and odds are you are really just kill stealing from your team for epeen. That's one of the biggest mistakes people do in the league. Why? You can only buy six items. You stealing gold is really just robbing your team and you cannot win a match 1v5.
2) You do not go for towers. You are roaming for kills only and you are the AD Carry in most matches that you're playing meaning you are the one who should be towering, not your AP carry or your support. They can take it down sure, but not nearly as fast as a well built ad carry.
Odds are you are playing for epeen not for match goals. ie, you're going for kills only. Not towers, not dragon/baron control etc. You need to learn to share to win the game. Twitch is a very kill steal type champion and more times that not looks like you're capping your team off at the knees.
Before you go off and use the "but I need gold" response, that's a poor argument. Your creep score should make it so that you have enough gold for full build before your 10th kill. Your CS in most games is really really low. You typically get 5cs per kill. That's really low by any standard for an ad carry.