I'm not even sure I would tell her that you miss her. I would just tell her that you wish her well and that you hope she finds what she's looking for. If you tell her that you're going to miss her, you will appear weak to her and she will picture you sitting at home pining away for her. Forget that!
In all honesty, when I saw H last time, I told him that I love him AND that I will miss him, then I went no contact again. I know I shouldn't have said that to him, but it happened. I did the best I could at the time. I just have to pick myself up, dust myself off and try again when I mess up. Just do your best, that's all you can do. Try to be as strong as you can be until she is out of sight and completely gone, then you can let your feelings out, go in the house and scream, whatever you need to do. When I left his house, I didn't cry until I was in my car and the door was shut...then, I lost it. He will never know that though. Good luck. We will all be pulling for you.
M:45 H:48 M:11 No kids BD:Sept'15 EA:Confirmed 1wk later PA: Oct'15 12 '15 2 wk R Just kidding, H wants NC 12 '15 H back w/OW 4 '15 R &still working on it