If you were speaking then こちらこそ alone would be fine, but since it is email, I think it would be best to add ご無沙汰しています。
This is to avoid the disconnect of "こちらこそ about what?"
If she is even a little 目上 or you just want to express added politeness change the います to おります。
Both of these versions can be modified with ...
There are a few different ways to express missing somebody.
Generally saying that you miss someone (hence, you want to see them). This is the most casual option available to you.
I miss you.
**I think this is your safest bet.
It's been awhile, I miss you!
This is a really strong missing; a ...