ADC: Tell me all the things

ADC tristana main, stalling out real bad in silver V. IDK what it is but i just can't get a decent winrate now and I don't understand why it's so hard to win a game now. Here's a couple replays from recent games I had in ranked, and i'm hoping someone more experienced can tell me what I can do better to carry my team.

http://www.replay.gg/search/na/Bootiegirl#2115436586 This game goes to mid-late game. I actually do great in the early laning phase, I get caught out in some ganks and that throws me off. Once the laning phase ends I just go to farming, creeps, camps, w/e I can so I can try to get more XP and items asap. Once kog maw really starts grouping with his team I start grouping with my team to counter it. I got some decent items and made it far enough to almost complete my bloodthirster (my build was IE>berserker boots>rapid fire cannon>domminick regards>BT; this is the typical build I run pretty much all the time on tristana).

We just couldn't win the team fights, even when i'm trying to avoid getting too close. I could also use a pointer on when I should know i'm strong enough to group with my team because it's not very clear to me. All I know is I at least need my first 3 big items done before I regularly stick with my team. Till then i'm pretty much farming camps, pushing bot as far as I think I can before I could die to a gank.

http://www.replay.gg/search/na/Bootiegirl#2115432511 This is an example of a game snowballing out of control. I fall behind but if i recall correctly I only die once in the laning phase, but my team is falling apart around me. Every lane is getting pushed in, turrets dived and destroyed. I feel like it's pretty much luck whether or not the game can get delayed long enough for me to get to late-game. I can't afford to go mid because i'd be splitting XP with the rest of my team while the enemy team is grouping, and until I get items I feel like a liability. But again, it feels like my team just can't hold, doesn't seem like I can really do anything about a lane getting fed (when it's not my own) and going around roaming to each lane and giving map control to the whole enemy team.

So here's a couple replays, if anyone can look through them and analyze what I could do better it'd be pretty awesome. I don't think I can really figure this out on my own so any help is greatly appreciated.