Fiora's kit was designed to make her movement's look fluid and calculated like an actual fencer, fiora might play completely different than riven but in same way that a riven can completely outskill you, so can fiora.
Her damage was nerfed into the ground, her cooldowns were vastly increased and her heal is literally 25-115 HP for vital hit, dependent on level. A lot of champions out damage her and she has a huge amount of outplay potential and there are several hard counter's for her that just make her useless.
Yes the fiora does have to make a decision, because all of her match ups are skill match ups or counter's to her she has the complete and total possibility of loosing a trade for 1 vital proc. It happens to me all the time, i'll go in for one and get instantly half healthed because it's easy to predict were i'm going to stop dashing and were i'm going, in addition she cannot use her W until the end of her dash, meaning AOE stuns will work on her if timed correctly like say from a lissandra. Finally if you need to walk out of the range of her passive then back in to reset the vital, forcing the fiora to have to deal with it being in a different spot. It's pretty frustrating when someone does that to me especially if it's someone i know i can't fight.
I honestly think you just have an aversion to her, i have an aversion to Renekton, i just can't stop myself from feeding and i feel like there is nothing i can do even if i play safe he's just gonna dive me.
Counter's to fiora that i'v seen: Renekton, Gangplank, Malphite, Tryndamere, Kayle, Ekko, Illaoi, Pantheon, Swain(lots of sustain, slows, roots), Quinn and Lissandra.
If the fiora is really good she can still win against those but those are all pretty hard matchups for her, the higher your ELO the more frustrating fighting fiora will get but if you want to have a chance those are the champs that i'v had trouble with while playing fiora.
The illaoi one i'll explain though, its a mind game with this one, you need to be able to bait out the Fiora's W and because she has no CC you don't have to worry about being stunned back or something. Fiora has to get really close to fight you and that's what Illaoi loves more than anything, the only thing the illaoi has to do is keep up with her while she's moving from vital to vital, you out sustain her as illaoi even if she does percent health damage, it won't bother illaoi as much due to her massive sustain.
I'v lost horribly to an illaoi 3 times as fiora but only beaten one once with her. I'v also stomped several fiora's into the ground with illaoi so i'm thinking this one is in illaoi's favor you just have to be careful early.