Suggest Treatment For Vaginal Itching

hello i am a 19 year old female, I have had vaginal itching and thick white discharge for some time now, i have been tested for a yeast infection more than once and the results have always been negative. The discharge often smells like feet it is better when i have just washed but later on in the day it will be the same again. The itching is on the outside of my vagina and nearly always on the left hand side near the top. I have been to my doctor about this 3 times now and still have no answers she thought it might be a skin condition and gave me an antiobiotic cream this worked for the first day or two but then the problem returned. I am sexually active but have had this long before i ever had sex so i know it is not an std. Is this a common thing? Does anybody have any idea why I cant get rid of it? i do eat a lot of sugar in my diet but surely it shouldnt affect me this badly?