at best

The impracticality of a psychology restricted to self-observation is shown by our inability to imagine how such a scientist could qualify as a pedagogue, psychiatrist, or applied psychologist. Unless one can use his introspective skill he can only meditate upon his own thoughts and feelings, and can only become what the practical psychologist often calls a "compulsive neurotic" or, at best, a philosophical thinker. Most of us feel that self-knowledge is worth the effort only when we can use it to understand, control, or "help" others – as we so often rationalize our intent to control.

Otto Rank, Psychology and the Soul (1950: 8)

What one can promise

One can promise actions, but not feel­ings, for the latter are involuntary. He who promises to love for­ ever or hate forever or be forever faithful to someone is promising something that is not in his power. He can, however, promise those actions that are usually the consequence of love, hatred, or faithfulness, but that can also spring from other motives: for there are several paths and motives to an action. A promise to love someone forever, then, means, "As long as I love you I will render unto you the actions of love; if I no longer love you, you will continue to receive the same actions from me, if for other mo­tives." Thus the illusion remains in the minds of one's fellow men that the love is unchanged and still the same.
One is promising that the semblance of love will endure, then, when without self-deception one vows everlasting love. 

Nietzsche, Human, All Too Human, 58

more dangerous

Socrates considered that to delude oneself that one possesses a virtue one does not possess is an illness bordering on madness: and such a delusion is certainly more dangerous than the opposite illusion of being the victim of a fault or a vice. For in the latter case it is at any rate possible one will become better; the former de­lusion, however, makes a man or an age daily worse – which in the present instance means more unjust. 

Nietzsche, On the Advantage and Disadvantage of History for Life §6