Shame it's all gone pear shaped between the two of you.
I was cofused by the title of this thread. 'casting up' is not an expression I've ever heard before.
I'm glad I'm not the only one who didn't know the expression. I googled it and found that it is an expression mainly used in Scotland. I'm a southerner so I guess it hasn't travelled down here.
Worriedson1, it seems to me that it is difficult for us to understand what might have caused the person to say what she did without knowing what the argument was about. It sounds as if the helper was angry about something else then used the "after all I've done" line in the heat of the argument.
I have certainly been in situations when two people have had completely opposite interpretations of the same events. For example, when I got engaged, it unfortunately co-incided with the time when the girl I shared an office with had just had a very painful break-up with the man she thought she was going to marry. (Everyone else could tell that was never going to happen, but that's another story.) I was very aware that my news would be "rubbing salt into the wounds" for her, and after the initial announcement, I was very careful never to bring up the subject of my forthcoming wedding. However, I couldn't stop other people asking me about it so inevitably she did sometimes hear conversations about it. One day she accused me of continuously going on about my wedding to deliberately upset her. I had definitely not done that, but clearly for her any mention of it by anyone was too much, and my replying to someone else's question counted as me "continuously going on about it". She was not aware of the many times I had wanted to make some reference to it and had not done so, to spare her feelings.
It may be that while from your point of view you have continually thanked the helper, they haven't noticed all the praise and don't feel they have been thanked enough. However, when people are angry and having an argument all kinds of things are said which are not really meant or not really true, so that may be what happened here too.