What is the paraphrase version of the line in Macbeth-- 'That which cries, “Thus thou must do,” if thou have it' mean?

Macbeth has sent Lady Macbeth a letter. In Act 1, Scene 5, she reads the letter out loud to herself. The letter tells her about how Macbeth met with the witches and what they said to him about one day becoming the Thane of Cawdor and eventually the King. She is quite moved by the news but doubts that her husband has what it takes to act decisively, boldly, and viscously. She says:

Glamis thou art, and Cawdor, and shalt be

What thou art promised. Yet do I fear thy nature;

It is too full o’ the milk of human kindness

To catch the nearest way. Thou wouldst be great;

Art not without ambition, but without

The illness should attend it. What thou wouldst highly,

That wouldst thou holily; wouldst not play false,

And yet wouldst wrongly win. Thou'ldst have, great Glamis,

That which cries, “Thus thou must do, if thou have it;

And that which rather thou dost fear to do

Than wishest should be undone.” Hie thee hither,

That I may pour my spirits in thine ear,

And chastise with the valor of my tongue

All that impedes thee from the golden round,

Which fate and metaphysical aid doth seem

To have thee crown'd withal.

Very knotty use of the English language there in bold face, for sure; no doubt it was a quote more familiar to people in Shakespeare's time than in ours. But what she is saying to her absent, but shortly-to-arrive husband, is this: "You need someone to tell you what have to do, and you shouldn't be afraid to do something to the point of wishing it never happened." Then she says, as befitting the quote: "Come on home, and I'll make sure to convince you to get what you were promised and what you so richly deserve."

Ah, what a woman!