I miss you terribly!

我多么思念你呀！

《新英汉大辞典》

I'll miss you terribly.

我将会很想念你。

youdao

Though I miss you terribly, I can feel your love across the miles.

我仍可感觉到你的爱穿越千里来到我的心中。

youdao

We all love you so much, and we will, I will, miss you terribly.

FORBES: My Aunt, Olive June Black, An Ohio Original -- In Memoriam