Inversions set yoga apart from other physical disciplines: Psychologically, they allow us to see things from another perspective. Emotionally, they guide the energy of the pelvis (the energy of creation and personal power) toward the heart center, enabling self-exploration and inner growth. Physically, they stimulate the immune and endocrine systems, thereby invigorating and nourishing the brain and the organs. When done correctly, inversions also release tension in the neck and the spine.
Because of their myriad benefits, Sirsasana (Headstand) and Sarvangasana (Shoulderstand) are considered to be the king and queen of asanas, respectively. Sirsasana develops our capacity for action (fire element) and enhances our ability to create (air element). Sarvangasana nurtures our capacity to stop doing and to become grounded (earth element), and it fosters our ability to be still and reflect (water element). Sirsasana makes us more alert and focused, while Sarvangasana makes us calm and receptive.
If you've been avoiding these poses for fear of falling, it might be time to rethink your decision. If you're strong enough, an experienced teacher will be able to guide you into these poses safely. If you don't yet have the strength, ask your teacher to tell you what poses will help you become stronger, and work on those until you're ready.
It takes a while to build up a practice of Sarvangasana and Sirsasana, however. Be patient with yourself and take the time to master them. If you do, you will reap their benefits for the rest of your life.