I think the reason we find it difficult to be stylish, is because we don't understand how to be stylish. We struggle interpreting our own Personal Style and we can't differentiate between fashion and style, or simply being fashionable while still being stylish...
As individuals we perceive style very differently from each other based on age, personal choices, gender, body shape, location, weather patterns, financial and perhaps lifestyle. We find it hard to be stylish, because everyone has a very different interpretation of what they think style is.
Because we don't understand the fundamentals of style and the importance of undergarments and proper tailoring, we copy what we see in magazines and on the runways, thinking we will achieve the same fabulous look, yet we do not know how to wear it to suit our individual body shape and personality. We try and overcompensate by wearing all the latest trends at the same time, not understanding that keeping it simple is much more elegant. By adding just one trend at a time, it still translates you know what is fashionable.
I also think another major factor is that we are lazy to dress up and don't make an effort most of the time. We simply can't be bothered. I know I am guilty of this many days...We think if we dress up we will be overdressed because nobody else does!
By feeling confident in what you are wearing, it will automatically translate as stylish, regardless of what society makes us believe style should be. Forget about what everyone else think style should be and only focus on what makes you feel fabulous and what you feel comfortable wearing. Make the effort. Remember you can never be overdressed! Don't worry so much what people might think or what other people are doing. Dress to suit yourself!