This question has already been answered: http://www.enotes.com/macbeth/discuss/do-macbeth-lady-macbeth-truly-love-each-other-116559
Before determining wether or not Lady Macbeth was constant in her love for Macbeth, you first have to determine the nature of her love to begin with. Lady Macbeth, though it is possible she may have felt some sort of romantic passion for him at some point in their relationship, is principally concerned with power. Every action she takes is to preserve, enhance, and expand Macbeth’s power and by proxy her own influence through his affluence (until, of course, her eventual demise). At Lady Macbeth’s point of demise, she is incapable of love or any real emotion at all that extends beyond the desperation of her predicament. So, did Lady Macbeth remain constant? If we accept that she never loved him to begin with, then yes -it remained at a constant absence. If we consider what may have happened before the beginning of Act I, perhaps -but throughout the duration of the comprehensive play the only real affection she affords is to power.