PCOS and TTC?

I am 21 and have been TTC for 19 months now, I have PCOS and feel like giving up. I was prescribed Metformin to help conceive but I saw another doctor and he told me it wouldn't help me conceive only help to lose weight. I am 14 stone and I know that doesn't help but there are women who are much heavier conceiving all over the world. Any suggestions on what may help me? I try to exercise but I have severe knee problems which doesn't help, and eat as healthy as I can! Does anyone have any suggestions to what may help me? :(