I'm in a realy tough spot...

Today she was very attentive and affectionate. It gives me a wanted, hopeful feeling inside. But it also puts me in a very hard spot. If I act all happy and smilly, she is going to think that all I am looking for is sex. And If I still look as sad as I still feel inside, she is going to think that there is no hope and grow more distant. How do I tell her that her effort is appreceated and welcome and also tell her that she is stil missing the cause of the sadness.

Let's say you had a knife stuck in your back. Now, there are two ways you could go about fixing the problem.
1. You could treat the symptoms by taking pain killers and wipe up the blood.
2. You could treat the cause by puling the knife out first. Then using the pain killers and wiping up the blood. Both of these solutions will give you immedate relief, but only one will cure the problem forever.

In order for me to start a real recovery I need her to first pull out that knife.
I need to know, in no uncertan terms, that it is over with him.
I need to know that she is never going to contact him again.
I need to know that if he calls or tries to contact her, that she will not respond. That she will willingly choose to stay with me.
I need to know that I am her knight in shining armor. That I mean to her what she means to me.

Then, and only then, will the pain killers (affection and sex) have a lasting effect twards recovery.

So how do I do it? How do I let her know that I love what she is doing, but it will not help as long as the knife is still there? How do I tell her that pretending that everything is OK will not fix anything? How do I do this without making her feel unappreceated?