I’m sorry for your loss OP. How long ago did you lose your partner? Your grief is very much coming through in your posts, so I am wondering if perhaps it is still quite recent, and this feeling is a big part of the grieving process.
People are trying to help with their comments. I had my eldest at 25 - at times over the years I’ve felt envious of those having their children in their late 30’s as I feel I missed out on my youth (I met my husband when I was 19, when I left uni I went straight into full time work, I never travelled, never had big nights out with friends, never did spontaneous trips etc) but then those who had their children later say to me at least I’m still young enjoy the freedom that is finally beginning to appear, whereas they are still in the trenches, so to speak. But with a number of chronic health conditions now, I can’t enjoy that freedom in quite the same way. Anyway, the point is, there is no perfect age to have children, and we almost all feel that loss of ourselves etc. I do get what you are saying though about it being harder with no partner, or even no ex in the picture - but you only have to read a few posts on here to realise that many women, even in relationships, are shouldering all the burden and their useless “partners” might as well not be here as they will never do anything with the kids!
As mentioned, I am married, but in the 21 years since having my eldest, I have had 1 weekend “off” where I went away with 2 friends in 2019 to a festival. That was only possible by planning it 9 months in advance, the kids being much older (they were 15, 12 and 8), and me making sure as much was done as possible before I left. I also had to arrange for my parents to be on “standby” if any help were needed….
No-one’s life is perfect. We all wish for something we think others have better. If I could switch situations with you, I actually would in a heartbeat (that sounds awful I know, but after years of emotional abuse and shouldering the burden of everything with the kids, that’s just how I feel) - it sounds like your partner actually pulled his weight and I wish I could give you that back.
I know it gets trotted out on here a lot, but some therapy/counselling could be really beneficial to help you work through your grief.