I understand what you say, totally.
I suppose there may have been a chance that she felt she had been pitted against you. Can't have been easy for either of you. I clearly remember one of the SW's saying 'Refuse to let your DM have DD, or you will lose DD and DM will control contact' .. and 'If DM applies she will get DD and you won't, tell her to back down'.. I didn't think about it rationally at the time, and given that we had had a very difficult relationship in the past, i made snap decisions. In the cold light of day DM would have allowed totally free open ended contact. Another thing to kick myself over :( They made me feel like i was pitted against my own mother.
I'm sorry to hear about situations such as yours, because i know women who have walked out and left DC's, then come back years later and created problems by saying 'you were forcefully taken from me, i didn't want to lose you' to ease their own guilt.
I don't know why people choose to self sabotage, then lay the blame at others' feet despite being given every aid, every opportunity.
Sure you can make mistakes...
Are you sure that she hasn't got this rotten view of you, and believes you 'won' her DS, and can't now bear to see him with someone else? Not saying that feeling is right, or unselfish, she should be able to swallow those feelings, to visit DS, even if she doesn't like you.
I'm sure you don't hang around during contact telling her what she's doing wrong ect? So she would have no reason to feel the way she does.
I've also known very manipulative kinship carers, (passive aggressives) and they make contact so difficult in the end the BP walks away and says 'I can't cope with this abuse i'll wait till DC's are old enough to come and see me, i'll explain then'
Very complex. Very complicated. And yes, each case is very sad.