Focus on keeping your spine straight. It is the job of the spine to keep the brain alert.
If you open the armpits, the brain becomes light. You cannot brood or become depressed.
When you are practicing, do not just do that for the sake of doing. Learn to reflect while you are practising. Make your mind and brain observe and relearn what you are doing. Doing is mechanical; learning is dynamic.
When you cannot hold the body still, you cannot hold the brain still. If you do not know the silence of the body, you cannot understand the silence of the mind. Action and silence have to go together. If there is action, there must also be silence. If there is silence, there can be conscious action and not just motion.