what is the meaning of it's all down hill from here
it means things will only get worse from this point on. I agree that it can be meant two ways...either it's going to get worse and go downhill or that you've made it over the hard part (the obstacle or hill) and it's going to be smooth and downhill  from here on.F you are an optimist then it is a good thing. If you are a pessimist then it is a bad thing. It most often is a double entendre, encompassing a hint of caution within the concept of coasting. Hard work will take you climbing up a hill, and once you reach the peak, gravity will slide you down the other side.