Where has my will power gone ?

I know all the things I should do. I know when I am doing something that I will regret later and yet I just have no control to stop myself.
offer me salad or sausage and I'll gor for the latter even though I know it is detrimental to what I want to achieve so badly.

how can I want to loose weight so badly but not be able to resist the bad choices that are presented along the way ?