Macbeth, as you rightly point out, has decided not to go ahead with the murder: he will, he says "go no further" in the business. Lady Macbeth is clearly frightened by this refusal to do the murder. And she immediately, without hesitation, attacks him as a coward:
Art thou afeard
To be the same in thine own act and valor
As thou art in desire?
Is he scared ("afeard"), she asks, to act and to do the things he desires to do? For a professional soldier, famed for his bravery, it must be a difficult thing to hear. But it doesn't work. She tries again:
When you durst do it, then you were a man,
And, to be more than what you were, you would
Be so much more the man.
Macbeth is no longer a man, she says. And then, trying to tempt him, she says that to be king ("more than what you were") would be to be much more of a man. But that doesn't work either. So she resorts to her big tactic:
I have given suck, and know
How tender ’tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this.
Macduff later tells us that Macbeth has no children. Yet Lady M has known what it's like to love your baby. But, she says, rather than break her promise, she'd have killed her own baby. And that changes Macbeth's mind. "If we should fail?" he says, back on board. Why? Why is this dead baby so emotional? Shakespeare never says. But it does the trick.