The little white lies you tell your lover 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 you deliberately mislead your lover; it's more of an accidental, even unconscious impulse.
You might get into the habit of lying by omission or fibbing a little out of what you think is kindness -- preserving their feelings for the sake of the relationship. Would you stop doing that if you knew that it didn't help the relationship at all? Even if it means hurting your lover's feelings, stark honesty is always the best policy. Don't be tactless, but do strive to tell them the truth. If you're having lunch with your ex, admit it! If you're feeling especially loving, say so! If you need some space for a few hours or days, speak up about it! That way, they always know where you stand, and where they stand in your heart. This all may be more easily suggested than carried out. If your lover finds themselves constantly feeling as if they're not getting the whole story, they might have to learn to 'speak your language' -- meaning, interpret the half-truths for what they really are.