Yikes, I don't know what to offer for advice... The closest I've run into is my D5 being upset that "her" bf wants to marry another Kindergartener. I told her she's beautiful and told her all the things I loved about her and we had a mother/daughter Saturday afternoon just she and I and went to Friendlys. I imagine that's a little simplistic for a 17 yr old but maybe, cheesy as it sounds, find a card for her and write all the things that are amazing to you about her in it. Sometimes teenagers (just remembering from having been one and from working with them) do better with all the expressions of affection when they can digest it by themselves first and then approach you afterward?