The little white lies your lover tells you aren't necessarily harmful in themselves; the problem is that they contribute to a general aura of confusion that affects your whole relationship. Any dishonesty, whether it's well-intentioned or even absent-minded -- meaning, absolutely unintentional -- just increases the general feeling between you that you can't exactly trust each other. This is not to say that your lover deliberately misleads you; it is more of an accidental, even unconscious impulse.
They might get into the habit of lying by omission or fibbing a little out of what they think is kindness -- preserving your feelings for the sake of the relationship. Would they stop doing that if they knew that it doesn't help the relationship at all? Even if it means hurting your feelings, stark honesty is always the best policy. They shouldn't be tactless, but you should encourage them to strive to tell you the truth. If they're having lunch with their ex, they should admit it! If they're feeling especially loving, they should say so! If they need some space for a few hours or days, they ought to speak up about it! That way, you always know where they stand, and where you stand in their heart. But this all may be more easily suggested than carried out. If you finds yourself constantly feeling as if you're not getting the whole story, you might have to learn to 'speak their language' -- meaning, interpret the half-truths for what they really are.