I don't think he really gets it

Both myself and my partner have children from previous relationship's and decided we would like to have another. He has 3 children I have 1 and I just don't feel done. You know like there's something missing but after 4+ years TTC with no logical reasoning as to why we're not having any luck. so we have said if it happens it happens if it doesn't well we will deal with that then. The thing is I don't think he really understands how much this would mean to me and unless that day comes he will never understand it unless he sees it with his own eyes