oldwomaninashoe I would absolutely agree with your thoughts on an 'only' child scenario.
My cousin and her DH split up when their only child was 3, but remained on reasonably civil terms for the sake of their DD.
Cousin met someone else about 5 years later - and DD could not handle it at all. She was used to having all her mother's attention and the pair of them being a tight little duo. DD was really jealous and difficult and awkward, so my cousin decided not to continue dating.
She tried dating a couple of times when DD was early/mid teens - only to get the same reaction. She and DD were really close - just the two of them in the house and DD resented my cousin having any life/friends of her own. She was foul on the couple of occasions her mother arranged to meet someone for dinner and threw a massive strop. Again my cousin gave up. She has spent her life simply being a working mum with no other needs but DD.
Cousin is now 54 and DD has finally got engaged to her own bf and they are buying a place together - but my cousin has resigned herself to the fact that she will be single for life. When I mentioned (tentatively) to DD that 'perhaps your mum might meet someone still' DD was really dismissive and said, 'Oh she's far too old! Who'd want to be with her? Besides - I don't want a 'stepdad' at my age!' and I felt really irritated at how selfish she still is about sharing her mother with anyone.
Admittedly my cousin has spoilt her somewhat - but I feel sad that she put her child first all those years (as you do) but is still not allowed to be a woman with feelings and needs of her own, rather than just DD's 'mother'.