In this line,
Tassā me aṭṭhamī ratti, yato taṇhā samūhatā.
I am puzzled by Tassā, which I take to be the feminine dative or genitive singular of “ta”, which would mean “to/for” or “of” , “her; that.”
I don’t see how this fits in the context. Various translators have rendered this “but now” or “this is.” Are they taking it as a dative of reference, “as regards this (the foregoing situation)”?
Hi there. 
This is a somewhat idiomatic use of Pāli, so bear with it. A fully literal English translation would sound rather stilted, although English has comparable constructions that can give a sense of what the grammar is doing.
The first stanza reads:
Four or five times
I left my dwelling.
I had failed to find peace of heart,
or any control over my mind.
This describes her earlier, troubled state.
The next line reads a little poetic. You could understand tassā me as something like:
“For that (same) me, this is the eighth night since craving was uprooted.”
In other words, tassā points back to the woman just described: “me, as I was then,” or “that same me.”
A roughly comparable English construction would be:
“I used to be a penniless vendor. Now there’s several businesses to that vendor’s name.”
Alternatively, Thag 1.62 can give you a hint of comparative usage:
We dwell alone in the wilderness,
“Ekakā mayaṁ araññe viharāma,
like a log dumped in a forest.
Apaviddhaṁva vanasmiṁ dārukaṁ;
Lots of people are jealous of me,
Tassa me bahukā pihayanti,
as beings in hell are of one going to heaven.
Nerayikā viya saggagāminan”ti.
Not the contrast of the previous usage, but again we have the sense of “With regards to me, there’s many people being jealous”.
Magnificent, magnificent! Precise clarification with English parallel, and a further example from Pali. Your explanation could not be bettered. Bravo!
