How to say in Arabic "Me but not you"

Hi friends!

Well, I have another interesting question...

How to say it right "me but not you"?

This is an exemple: أنا كتبت هذه الرسالة لكن لا أنت
It means "I wrote this letter but not you"!
How may I say in the end of this sentence? "me but not you" الا أنت or there is any proper form to express this menanig?

I'm not sure whether I understood your question correctly or not. 

Did you mean "I'm the one who wrote this letter not you"? Then you can say "أنا من كتب هذه الرسالة ولست أنت"

If you meant "I wrote a letter but you didn't" then you can say "أنا كتبت رسالة ولكنك لم تفعل/تكتب "

If you meant another sentence, let me know and I'll translate it.