What will make you feel worse? Telling your therapist the truth, even though she might be a little disappointed, or having a lie between you that you now need to protect? My guess is that initially you'd feel worse telling the truth, but in the long run you'd feel a lot worse because of the lie. At least I know that's the way I would be.

What about telling her that you tried, but you weren't able to do it *yet*. Or you weren't able to do it *this time*. Then tell her that her calling you was really nice and you appreciate it, and that you hope that next time that you'll actually be able to follow through. Try not to feel too bad. Learning to be assertive is not an easy process at all.