You intially are able to break up the Jin's static defense lines, as your army is wont to do. However, because they have an unbroken route straight back into the heart of their territory, reinforcements are able to arrive unimpeded. Eventually, the number disparity is just too large, and your army is overwhelemd.
You chose a non-historical decision! In real life, Genghis Khan split his forces in an attempt to negate the numbers advantage that the Jin had. Read more about how your decisions differed here.