After reading the initial post I thought of digging first. Then someone mentioned yoiu might be lying on a rock.
Well rocks can be a great way to sharpen the knife. So I would sharpen the knife on the rock and go to work on the tree (after propping it up on more rocks so that it doesn't keep weighing down on you as I hack pieces of the tree out.
Cutting your leg off just seems stupid. Eventually we will have to move and head back towards civilization. And dragging you back just doesn't sound like the smarter thing to have to do.