Focusing on a visual object

I was often discouraged in the past years attempting shamatha meditation, because it seemed fruitless. However, I noticed recently that if I focus on the breath, I have the impression that I am unwavering in my concentration. But, if I use a visual object, as an image of the Buddha, the image tends to constantly shift and alter in my mind's eye. Could this be a sign that am deluding myself when using the breath as object? Would I be better using a visual object to be sure I am really doing it right?