Losing Prepregnancy weight

Mummies, is it okay to fast 3 days while BF? I couldn't shed the last 8kg and I'm feeling so depressed... i got to admit I eat little but I drink a cup of zero sugar milktea (my source of happy food) to keep my sanity.. On the other hand, I am feeling so sad as I couldnt fit into my old clothing. Any recommendations to lose weight fast?