All Beings are our Mothers?

I have often heard the argument in Tibetan traditions that one must learn to see all sentient beings as one's mother, particularly as we have all been in this endless cycle of samsara...and somehow through all that time, we must have all mothered each other at some point. I find it interesting that mothering should be the paradigm for Tibetans, and I get stuck here. Even though I have been mothered, in fact, I don't recall ever having been a mother, and I don't think I even have what is called a maternal instinct within me. There are times when I simply find it hard to relate to the notion of loving all things unconditionally as a mother would a child. It is as though I am looking for a somewhat easier metaphor to work with, yet cannot find one at all.
   As much as I do value the metaphors of all sentient beings having been one's mother, I also feel that the maternal loving feeling we give and receive is never sufficient. Why do I say that? I say it because all those maternal feelings are skilful means to soften the heart, but they don't always necessarily constitute deepest wisdom. To give an example: we have all come across examples of mothers who overprotect their children or even develop unhealthy attachments to them, and vice versa. In fact, most psychology of the twentieth century is concerned just with this notion of maternal bonds gone awry! It leads me to wonder, is Mother a singular quality after all, or are there as many styles of mothering as there are mothers?