Stop beating up on yourself. If you had bought her a gift, or done anything, it would have set you back to the starting line. Like I said, it sounds as if you have been taught or conditioned to believe you had to do something big for her. I promise you that she noticed, and it did a lot more than if you had sent her something. You would have been lucky if you'd even received a thank you. But you would not have made any brownie points with her.
Remember, this all feels like you are doing the opposite. That's b/c a WW is not going to respond to what your feelings are saying to do.
And for goodness sake, don't break down today and call her out of your guilt. In fact, don't have guilt over it. You are the only one who can choose to feel guilty or not.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!