The quotation is from Act III of "The Relapse: or, Virtue in Danger: A Comedy" and is spoken by the widow Berinthia to Amanda, whose husband has been unfaithful.
Here's the passage:
Berinthia: What in the Name of Jove's the matter with you?
Amanda: The matter, Berinthia! I'm almost mad, I'm plagu'd to death.
Berinthia: Who is it that plagues you?
Amanda: Who do you think shou'd plague a Wife, but her Husband?
Berinthia: O ho, is it come to that? We shall have you wish yourself a Widow by and by.
Amanda: Wou'd I were any thing but what I am! A base ungrateful Man, after what I have done for him, to use me thus!
Berinthia: What, he has been ogling now, I'll warrant you?
Amanad: Yes, he has been ogling.
Berinthia: And so you are jealous? Is that all?
Amanda: That all! Is jealousy then nothing?
Berinthia: It shou'd be nothing, if I were in your Case.
Amanda: Why, what wou'd you do?
Berinthia: I'd cure myself.
Amanda: How?
Berinthia: Let Blood in the fond Vein: Care as little for my Husband as he did for me.
Amanda: That would not stop his Course.
Berinthia: Nor nothing else, when the Wind's in the warm Corner. Look you, Amanda, you may build Castles in the Air, and fume, and fret, and grow thin and lean, and pale and ugly, if you please. But I tell you, no Man worth having is true to his Wife, or can be true to his Wife, or ever was, or ever will be so.