WHAT IS UP WITH THIS?

so a little back story we are high school sweethearts been together for 19 years and married for 12. we have a great 6 yr old and been wanting to have a second little one. so we have been trying for over a year now. ever since we started my husband has watched me do all the temping and testing and we will have sex days before my fertile week but during that week nothing he says he not feeling it, I'm putting to much pressure on him, he's tired then after I have ovulated he wants to have sex agian but only once. so that's once be fore and once after but never during. so what I want to know is it just because we have been together so long or he's just not interested anymore and I should just take it that we are an old married couple and just enjoy our late 30s PLEASE HELP WITH advice or opinions