Yes I have regrets. My son is an only child, not by choice. DH and I hoped to have at least two, possibly three or four; however, due to health/fertility issues, it wasn’t to be. I love being a mom and I’m very grateful for DS; he has given me so much joy. I wish he could’ve had a sibling (I have 5 and we’re all close), but it’s all he’s known.
OP, I think your feelings are natural and will pass with time. We are at the same stage in life (though about 10 years older). DS is 22 and moved out (to another state) last summer, after graduating from college. He is living his own life, as it should be, and doing really well, so we are happy for him. We miss him of course, but have adjusted to the new normal. If someday we have grandkids, I’d be thrilled but am not taking that for granted.