Joe,
In the 17thc version your stanza runs thus:-

Tho' I am forsaken, yet she is forsworn,
Yet she is mistaken if she think that I'll mourn,
I'll set as slightly by her as e'er she did me,
And for ever will deny her, let her go, farewel she.

This may throw some light on your interpretation, though it is quite likely there were even earlier versions.

Note the last line which eventually evolved into a separate lament.
Steve