Fiora's mechanics

I recently started playing fiora, in sence a decently strong champ, but I feel she is missing something. Alot of players say shes very unviable and broken in the sence of escaping when needed. When I played Fiora i have a difficult time in early game against champs like darius,singed and renekton(tanky attackers), especially when they are building armor. I feel that it could be my fault for not adapting well enough to her mechanics. I am aware of all builds relating her online, but I feel like last whisper is not enough attack or pen, and I personally dont want to buy top notch boots to run away for an un-winnable situation every time. I do good with fiora at random it's very different every match. I feel she is very good under certain team conditions and late team fights, but as an initiator im not sure. I feel that i should build health/damage items like forzen mallet but then i miss out on damage and pen for those higher health champs. Personally some of you might think im completely wrong but just tell me how it is and what i should do. I want to make her my main but im not sure how to make her more stable -Gnov