P.M.
You are asking such a big question, A.. It's actually several big questions:
How do your memories and previous history affect your sense of completion now? What constitutes "enough?" Since another child will have a huge impact on all family members, how do the various members balance their wishes and needs? How can you most responsibly make that choice for your sons?
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. And, contrary to your observation that many women "have assurance that they are done," among all my lifelong acquaintances, there are far more women who would gladly HAVE more babies, but realize their families 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.)
And because getting pregnant is something of an uncertainty for you, there's also the dynamic of "scarcity increasing desire," one of the emotional triggers we all have to deal with in all areas of life. That struggle might be further intensified by your husband's wish to stop with two.
How to cope? Can you alter your longing? Well, it will probably always be there to some degree, because Nature has programmed it in, and because you always wanted another sibling. But you CAN shift your attention away from what you don't have to what you do have. Ah, blessings! You CAN decide to stop dwelling on your sense of incompleteness. This is the "secret" to contentment of all the women I know who would gladly be pregnant again, and again, and again…. 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.
I wish you peace, A.. I hope you find a way to accommodate the choices open to you, and be happy.