Hugs, JennP. I think the call to responsible parenthood is one of the most difficult to live out. So, we're supposed to be generous, not selfish, but so often, it is so difficult to determine what is generous and what is selfish. We often think of having children as generous and holding back from creating more life as selfish, and that is sometimes the case, but sometimes the opposite is true. Sometimes, it's generous to acknowledge your limits so that you can give generously or selfish to just want to hold another tiny baby or to not wish to use sexual self-control.
One thing that is abundantly clear in our Catholic faith is that we always maintain an openness to life. We are called to try our best to discern and follow what we believe to be God's call for our family, and usually he cooperates with us, creating children with us in the fertile time and not in the infertile time, but sometimes, he has other plans for us, and we need to seek to be open to that. Some couples struggle with unplanned pregnancies, and others struggle with infertility. It's not all in our hands, and we shouldn't pretend that it is.
What does God want for your family? Family planning is a part of our vocation, not something we just decide on our own. Is God calling you to have another child? Are there some things in your life that he is asking you to work out first? Is he telling you that you have a lot going on right now and need to wait for some things to change before you can even consider having another child? Is he asking you to just raise the three children that you already have, and that is all he is calling you to have? It takes prayer and openness to try to answer these questions. Then, it takes trust and self-control to seek to follow God's will while maintaining openness to life. Then comes the need to be open to things changing. You may find that God isn't calling you to have another right now, but some months or years down the road, that changes, or you may find that over time, he takes away your feelings of wanting one more and gives you more of a sense of peace with the children you have.
That is one of the great things about Natural Family Planning. It's not doesn't have to be permanent or even long-term, and it is always open. Your discernment is really only for right now (though you may have an overarching plan); you can always change later if you feel a call. It's not set in stone. If you haven't considered the Church's teaching on Natural Family Planning, I would encourage you to look into it. I think a lot of times, our culture focuses on the negatives of NFP when there really is a lot positive about it that we don't hear about very much: health for us and any children we conceive and nurse, spiritual openness, self-control, communication between spouses, environment, and more. It's not just the Church trying to tell us what we can and can't do, but really, our Holy Mother Church seeking our best interests in so many ways. If you haven't ever considered it this way, I encourage you to take a look.