OP, I don't think you're being unreasonable at all.
I share your sadness. I have a nice life - a good job, a nice house, good friends, a cute dog even, but I am now 44, single and childless. This may be a good life, and most of the time I thoroughly enjoy it, but it is not the life I wanted or expected I would have. Sometimes it feels as though my life is second best to the one I wanted.
I still have my parents, though I have little contact with them due to borderline emotional abuse, and although I have a sibling, we aren't close and he has no children either.
Oh, and in case anyone is wondering, yes, tried the AI route alone (for me, it was important to know I'd tried everything, but it wasn't an easy decision). Even had an accidental pregnancy (two miscarriages). Fostering is not an option for the fulltime employed.
I don't need counselling, I have no unresolved issues about my situation. It does sometimes make me sad, however. I think that's understandable.
So no, OP, you are not BU in the slightest, and you're not wallowing. Thank you for confirming that my feelings are not unreasonable as well.