It's SO normal to feel that twinge of longing. Nature designs that into us childbearers, mind and body, 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 (I'm now in my 60's and stopped with one daughter, myself), 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 indefinitely sustain a rising human population – many scientists and social scientists are seriously concerned that we've already reached a tipping point that will have severe consequences for future generations.
How to cope? Can we alter our longings? Well, Baby Fever will probably always be there to some degree, rising and falling with your hormones, because it's programmed in. But we CAN shift our attention away from what we don't have to what we do have. We CAN decide to stop dwelling on what we 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 the families they have, of whatever size. Contentment is a state of mind, and it is mostly deliberate.
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 our longing for another baby is one of the things our minds can do, if we decide to. We can just notice it and keep on keeping on. It is a choice.
A few times when I just HAD to have contact with babies, I offered help to new moms in my circle. They loved it, and so did I.