Not crazy, just normal. This is an important question: What constitutes "enough?" Since another child will have a significant impact on all family members, does considering that choice in light of what's best for your whole family make an "emotional" difference for you?
It's SO normal to feel that twinge of longing. Nature, and culture, and our individual histories, conspire to make us willing to endure pregnancy, childbirth, the demands of babies, lack of sleep, etc. So, if that urge wasn't strong in us, the human race would probably have died out long ago. Many, many women, probably most of us, 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? Longing will probably always be there to some degree, rising and falling with your hormones and social pressures. It can become a huge part of our self-identities, especially as we gradually become consumed by the needs of our children.
But you CAN shift your attention. Ah, the many, many blessing of a smaller family! You CAN decide not to dwell 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 to a very large degree, it can be chosen. I chose to stop with one daughter, for good, practical reasons. I never regretted the babies I didn't have, because I took such delight in the one I did have.
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: There is that longing and regret many women feel for not having one, or two, or three more children. And, contrary to the "common wisdom," I have also known quite a few moms (and dads) in my several decades of life who ended up regretting having more children. Health problems in parent or child, a change of circumstance, loss of job or home, even a neurologically challenged child have been contributing factors. These moms and dads loved all their children, of course, but nevertheless, in quiet, honest moments, wished that one 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, J.. I hope you find a way to accommodate the choices open to you, and be happy.