Hey! I was exactly like you, but a lot heavier (size 22). I also have pcos. I would do half hearted diets but never stick to them. I went to the doctors a couple of months ago about my bad back and he gave me a right lecture as I'd put on 3 stone since I'd seen him last (2 years previously). I could tell he was really annoyed and probably a bit disgusted. That motivated me to go on a proper diet.
I used the app nutracheck (like myfitness pal but I found it easier to use) to track my calories and I've lost 3 stone so far. I've still got another 4 to go before hitting the "healthy" BMI range (I know!!), but now I've started losing weight I feel aI can do it.. (will definitely stop/eat to maintain when I reach my target though!)
I think the motivation is the key, and the doctor telling me about diabetes and my knees giving out and that was causing my bad back etc (to scare me), worked!
The things I've found most useful -
At the beginning I weighed everything to add to the app, and this really helped my portion control.
I found for me that rice cakes are a saviour, either salt & vinegar (or plain etc) for when you want a savoury snack, or the caramel or chocolate ones for when you fancy a biscuit etc.
Laughing cow light or extra light is a great alternative to proper cheese and v low calorie. I have it for lunch with ryvita and salad etc.
I'm not eating pasta, potatoes, bread or rice at the moment, I think this has really helped. Instead I'm eating pureed cauliflower instead of mashed potato, or beans or lentils or quinoa as a "carb" substitute.
I'm just eating LOTS of veg and quorn and pulses etc really... I'm vegetarian, but even if you're not, quorn is a great lower calorie alternative to meat.
My main thing is portion control, and to not eat "treat" foods as the norm (ie pizza/ice cream etc). These are for special occasions (when the diet is over!). It is working for me (fingers crossed), but everyone is different. I would be up for a pcos diet thread!