I can totally understand why you are torn on this. As someone who had DCs quite young these are my thoughts:
I had health complications after DC1’s birth which gave left me low on energy and with pain, depression etc. Obviously the fact I went on to have more DCs it wasn’t as bad as it could have been, but consider the physical toll pregnancy, birth and the possible after effects could have on your ability to solo parent. I had XH around but he wasn’t hugely helpful. But I’d have really struggled alone.
As you approach menopause and your oestrogen levels drop you can lose those nurturing feelings. Whether its because for many of us we’ve already put in years of hard work, or its a completely hormone driven shift, but a lot of us find that in our 50s we just want to be left in peace! Caring for a clingy child at my stage of life sounds hellish.
Having been a single parent to older children (mine were 6,8 & 12 when I split with their dad) wasn’t actually that hard. I did get one night a week off to go out with friends or on dates and I got financial support from their dad. You sound like you’d be ok for money, so as long as you have some practical support to give you a break you'd be fine.
However, meeting a new man late 40/early 50s is going to be haaard! Dating has changed a lot lately and many of the men on apps are married, just after sex, looking for a 3rd for their ENM open relationship, just wanting to chat and/or send photos. They’re grim.
I have met many men and would honestly say if you’ve got a decent one, keep hold of him. Of course there’s nothing to say that he’ll be the one forever but a bird in the hand is worth two in the bush.
Final thoughts FWIW, your DP needs to be 100% clear on the level of sacrifice you’re making for him. It may be that your resentment tarnishes the relationship or it may be that he realises how much you love him to have chosen this life. He needs to ask himself if he can really step up. I’d hate to imagine you choose him, only for him to waltz off with some 27 year old woman from the gym in a couple of years and have a baby with her.
To kind-of-quote Carrie from Sex and the City, as I can’t find the actual quote, you need to be clear on the answer to the hardest question a woman has to ask “will you love me enough to make up for the baby I didn’t have?”