Something equivalent to “I’m sorry to see you leave, but I’m glad to watch you go. I’ll miss you.”
Thanks!
Ça me fait de la peine de te voir partir, mais il est bon de te voir voler de tes propres ailes. Tu vas me manquer.
Literally: “I am sad to see you go, but it is nice to see you fly by your own wings. I will miss you.” French is a language built on images.
Elly, native speaker, french soviet cannukistan. 
I think we can make it a little shorter:
Desolé de te voir partir, mais heureux de te voir voler. Tu vas me manquer.
Thanks to you both.
This doesn’t sound quite right. Just saying “voler” is insufficient in this context. Elenfair’s suggestion is better.