It's SO normal to feel that twinge of longing. Nature designs that into us childbearers (so we'll be willing to endure pregnancy, childbirth, the demands of babies, lack of sleep, etc.) If that urge wasn't strong in us, the human race would probably have died out long ago. Many, many women stop having babies while still longing for more. Among all my lifelong acquaintances, there are far more women who would gladly HAVE more children, but realize their families and/or circumstances can't reasonably accommodate them. (Nor can the world sustain the rising human population for much longer – many scientists and social scientists worry that we've already reached a tipping point.)
How to cope? Can you alter your longing? Well, it will probably always be there to some degree, rising and falling with your hormones and noticing of larger families, because Nature has programmed it in. But you CAN shift your attention away from what you don't have to what you do have. Ah, the many, many blessing of a smaller family! You CAN decide to stop dwelling on what you don't have. This is the "secret" to contentment of all the women I know who would gladly be pregnant again, and again, and again…, and yet are content and joyful with what they do have. Contentment is a state of mind.
And our minds can override just about anything our bodies tell us. Or else people wouldn't be able to to diet, to run marathons, to overcome crippling accidents, to surrender needed sleep to meet the needs of a new baby, to quit addictive habits. Choosing to override your longing for another baby is one of the things your mind can do, if you decide to. It's a choice.
One final observation: contrary to the "common wisdom," I have known quite a few moms and dads in my several decades of life who ended up deeply regretting having more children. Health problems in parent or child, a change of circumstance, loss of job or home, even a "difficult" child have been contributing factors. These moms and dads "loved" all their children, of course, but nevertheless wished that one (or more) of those children had never been born – a terrible emotional burden for the parents, and sometimes for the child, as well.
I wish you peace, Christina. I hope you find a way to accommodate the choices open to you, and be happy.