Fiora help
So I'm a noob, and so far have been playing mostly mid mages like aurelion sol and syndra. Recently I decided to try an AD character so I picked out and bought Fiora for top lane.
I've been doing alright with Fiora, using a good recent build and everything. However, I always struggle vs characters that can outsustain me.
For example, no matter how much I hit nasus on the marks he doesn't care, he just regens, and when he doesn't h can take me on full health, while being on half health himself. Other characters that I struggle with are ranged - especially teemo, he just keeps poisoning me and quickly bringing me down to low health, forcing me to recall, so I'm forced to sit back and get CS under turret some of which he can still deny.
My own sustain consists of either 3 potions or a Doran's Blade and 1 potion. The 3 potions run out way too fast, whereas the Doran's blade has so little lifesteal I need to kill like 40 cs to regen half health - which usually isn't possible because they bully me out of CSing and I can't even get close to their creeps without danger of getting poisoned. Doran's blade works fine vs melee tanks, but even they can afford to stand near their creeps and damage me whenever I try to last hit.
This gets even worse when their jungler ganks me all the time, whereas my jungler never ganks top (My friend says top is too difficult and he would much rather get fed at easy gank lanes: bot and mid).
Any advice on how to play against characters that out sustain you and/or can do so much damage to you via range? I've already tried going aggressive and attacking them a lot, but the problem is, most of them can beat me in a straight up fight even if I hit most of the vitals.