Try placing a rolled-up mat under your heels. This will reduce the amount of ankle dorsiflexion (in which your feet reach toward your shin) needed and will allow you to find more ease in the posture.
Start by tightly rolling up about half the length of your mat (less if your mat is really thick; more if it’s thin). Place your feet together, with your heels atop the roll and the balls of your feet on the floor. Spread your toes to widen the base of support.