Don't be discouraged, I think they will become easier as you lose weight.
I wasn't able to correctly do lunges until I had lost a lot of weight. It was a balance issue when I was heavier -- I just couldn't balance my weight in order to use correct form. I think I was about 180 - 190 when I could finally do them, though they were still insanely hard!
If I put on a 100 pound backpack now and tried to do lunges, I'd fall right over! So if they're too hard now, wait until you lose some more weight and then try again. In the meantime, focus on other leg exercises that don't require quite so much balance.