Having trouble spotting on your pirouette? It happens. We lose our focus, we get distracted during the turn. Sometimes it's hard to actually see something and hold our focus, especially for multiple turns.
There are teachers who don't believe in spotting! Sounds crazy to those of us raised on it but it's true. For these teachers, it's the back that turns. They ask their students to turn with a broad back - an awesome suggestion but not the best way to do multiple turns, in my opinion.
But if you're having trouble holding a gaze and the "back turning" doesn't work for you, consider turning your bun or the back of your head. The ultimate goal of any turn is to have a sharp head lead the rest of the body. If you can somehow isolate the head and keep a loose neck, your body will follow.
So, assuming you have a bun that's at the back of your head and not on top, try to aim it at the mirror. Hold no tension in your neck and shoulders and let your torso and hips follow along. You may find this gets your head moving faster, rather than trying to glue your eyes on something in the mirror.