By definition, self-restraints, the niyamas are personally challenging. Tapas might be the one that brings up the most anxiety. Often translated as austerity (severe, stern), sometimes as blazing, burning or consumed by heat - it just doesn't sound like a lot of fun, right? But tapas is a practice that can take us to the tiptop of what we most envision for our lives in Yoga.
Tapas is a way of awakening and directing the abundance of energy that lies within us. It takes intensity of discipline and vision to do so. Donna Farhi describes it as "burning enthusiasm." Cultivating this energy allows us to move toward our goal of a fulfilled life of meaning. We focus on the things that are important, lesser things no longer matter and obstacles on the path are burned away.
The courage, effort and attention to building the habits and disciplines that move us toward a life of meaning, balance, service and love is tapas. It builds strength in body, mind, spirit and character. It isn't always easy! Change is hard. It's easier to slack off, do it the old way, or not at all. It's easy to be distracted by trivial things, the drama of others' lives or the latest trinket. Watching our thoughts and actions, consciously making decisions about what is truly important and then taking loving action that supports those important things - that is what creates a life of simple joys, meaning and fulfillment.
We can take little steps. Consistently doing something that is challenging or doing an "ordinary" thing with joyful and focused attention. We build our habits a bit at a time and they get stronger and easier. Going through the fire of tapas is worth the pure clarity on the other side!
References: Yoga Mind, Body & Spirit by Donna Farhi, Light on Yoga, B.K.S.Iyengar, Yoga for Dummies by Georg Feuerstein, GLYTA notes