I've been there OP and it's really hard.
Prior to having our DS both DH and I were pretty sure we only ever wanted one child and due to a very bad pregnancy and a very hard 12 months following the birth we didn't waiver.
However, when DS was about 15 months old I started to get broody and tentatively bought up the subject of having another baby and DH was having none of it. I let it go for a few months and then mentioned it again but he was positive he didn't want another. I told him my reasons why I wanted another which were primarily that I wanted DS to have a sibling but it made no difference.
I started to find myself getting upset about it (though hiding it from DH) because I just didn't feel 'done' and the thought of never being pregnant again or having another baby made me feel very sad.
I continued to bring it up in conversation every now and then and DH said he'd have another one if it mattered so much to me but I wanted him to want it too so didn't take him up on his offer.
Almost a year after I first mentioned it I'd had a really bad day at work (a baby had died) and when I got home all my deep upset cane pouring out and we had a really deep conversation about having another, it was probably the most honest we'd ever been with each other and the end result was that he agreed that we would try for DC2 and he promised me it was something he wanted to. I fell pregnant within two months he but it unfortunately ended in a miscarriage at about 8 weeks. DH and I were both upset, he said he couldn't go through it again and then went back to his stance of not wanting another baby. It was a really difficult time as I felt he'd snatched away something that was really important to me. I think it was just the upset that made him feel like that though as a few months after the miscarriage he told me he wanted to try again for DC2. My head was all over the place with him changing his mind.
Upon talking about the issue DH said the thing that was holding him back was because he felt that he was too old to go beck to square one even though he'd only just turned 34. He said he really did want to try again for DC2 but that if it hadn't happened in 12 months then we'd stop trying as turning 35 would be his cut off point. It took us about 10 cycles before I fell pregnant and I'm now almost 16 weeks gone. Thankfully it happened before he turned 35 as I'm very confident that once he got that mile stone he would have put an end to TTC.
I can understand why in his mid 40s your DH doesn't want to start all over again but that doesn't make it any easier for you
At one point during DHs refusal to have another I really tried to make peace with the fact it wasn't going to happen as I knew that his wishes to stop at just one child were equally valid as my wish to have another and I couldn't force him to want or have another baby.
Have you spoken to him openly about how desperately you want this? I think when I used to mention it every few months my DH just thought I was having general musings and that I was just going through a phase but when I really let him in about how much having a second child meant to me I think it came as a shock to him and he realised it wasn't something that was going to go away.
Some MN members talk about 'Deal Breakers' in terms of whether a woman who desperately wants another child is prepared to stay with a man who doesn't or whether the resentment would just be too much. I know I would never have left my DH over the issue, even if it meant me sacrificing my desire to have a second baby, but I'm not sure how it would have affected out relationship.
Huge sympathies OP as it's a really difficult and upsetting situation to be in 