Is it moral to tell white lies in a relationship?

If one's standard of value is man's life qua man, then the question, "Is it moral to do such and such," is the same as asking how the action will benefit or harm one's own life as a rational being, both immediately and longer range. Objectivism regards honesty as one of the essential virtues for man's life, and "white lies" (lying to placate others' feelings) as a form of dishonesty. This doesn't mean that one must be unnecessarily brutal in one's honesty toward others; one can strive to be tactful or perhaps merely non-committal toward others in situations where diplomacy may be warranted and will not be misunderstood as approval of something that one does not approve of. It's hard to see, however, how a romantic relationship could have any hope at all in the long run if the partners are anything less than totally honest and open with each other.

There is a very clear excerpt in OPAR concerning white lies in Chap. 8, near the end of the section on "Honesty as the Rejection of Unreality" (p. 274):

We can now deal summarily with the issue of "white lies." The ethical status of a lie is not affected by the identity of its intended beneficiary. A lie that undertakes to protect other men from the facts represents the same antireality principle as the con-man variety; it is just as immoral and just as impractical.... an improper practice is not improved by attaching to it an altruistic justification. If anything, the latter merely compounds the evil. It removes the liar a step further from reality.

For additional concretization, there are two scenes in Atlas Shrugged concerning white lies. In Part III, Chapter II, Dagny is describing how easily a narrow gage rail track could be built in Galt's Gulch to carry ore down from Francisco's copper mine. Dagny wishes she wouldn't have to hear about disasters in the outside world which she would want to devote her time and energy toward alleviating, but Galt tells her she would have to hear about them, fully. She thinks it over and responds:

She thought that no man of the outer world would have said this to her at this moment—she thought of the world's code that worshipped white lies as an act of mercy—she felt a stab of revulsion against that code, suddenly seeing its full ugliness for the first time....

The other scene is in Part III, Chapter III, where Rearden is explaining to Dagny:

People think that a liar gains a victory over his victim. What I've learned is that a lie is an act of self-abdication, because one surrenders one's reality to the person to whom one lies, making that person one's master, condemning oneself from then on to faking the sort of reality that person's view requires to be faked. ... there are no white lies, there, is only the blackest of destruction, and a white lie is the blackest of all. I was still faking reality, and it had the inexorable result: instead of protection, it brought you a more terrible kind of ordeal....

If the questioner needs to realign his girfriend's impression of him, he should do so. From the description of the relationship, however, he need not be brutal about it (unless she reacts blatantly irrationally). There is apparently so much about her that he still values.