Can't do headstand

I mastered the tripod but I can't do headstand, which is the second progression. 

I feel that I simply cannot lift my hips upward in a controlled motion like it shows in the video. What should I work on to overcome this?

I recommend starting with your back against a wall. Doing so will allow you to lift your hips and legs without worrying about toppling over. Once you feel more comfortable you can start to distance yourself from the wall. I also suggest submitting a video of your tripod for review. We might be able to help you refine your position so that moving onto a headstand won't be quite as daunting.