Question on Improving my Gameplay.

I'm a Silver 3 who recently finished provisionals right after the rewards were handed out. (Yes I did not get it, I regret why I didn't finish provisionals earlier) So the thing is, I pick a role, game starts, and I do have a good early game. I like to play Azir, Zed, Yasuo for Mid. Gankplank, Fiora, Jax, Ryze for Top. What I mean by good early game is I do good harass, I'm not killed in a gank (Unless I'm really off and tired) Get couple kills off, and get good cs.

But it problem always starts when it gets into late game. I really don't want to blame my team for it - I have that thought "Cooperation and Carrying the team is part of your skills" But I seem to have at least a lane feed hard - the enemy gets fed even harder than me in like matter of seconds - and it's just that enemy kills me always.
After that single person gets fed, I get killed, the whole team gets killed, and all of a sudden my KDA is like 7/9/6 (it was like 7/2/2) and enemy Mid-Laner who was 0/7/2 is now like 9/8/4. (For example, in my last game with Veigar, I picked up First blood and the next 2 kills alone and I was ahead a lot. But then Sejuani on the enemy was like 12/2/4 and Fizz who was 0/3/0 finished game with 20/20/9 and I finished game with 13/20/8.) Is it because I play champions with strong early game, is it that I need to roam more frequently, or is some reason that I don't know? I really don't want to believe that the fault is in my team mates, and I ask you guys if the fault is in me, and how can I fix it? I really want to improve my Gameplay so please help me and send me some good tips.