I need to drop 5 dress sizes, can anyone help?

I have 4 beautiful children, aged between 7 & 3, but unfortunately each of them was born by c-section and born in quick succession of each other (14 months between first 2, 11 months between the 2nd and 3rd and under 2 years between the 3rd and 4th). Each time I gained weight and never lost it again. I'm now left 5 dress sizes too big and I just can't shift it. I don't eat particularly loads (although I probably eat alot of the wrong things) I have joined a gym but because I'm so unfit I find I'm exhausted after only 10minutes on the bike and can only manage 6minutes of walking on the treadmill and I just feel like I'm not acheiving anything. I also have polycystic ovaries syndrome and I'm told that this makes it harder to loose weight, but also increases the chances of serious health problems if I don't (a visious circle) I'm so, so fed up and depressed with being fat, but despite how hard I try I just can't do it. Can anyone help? Please, please please?