One sentence: I’ve been writing the letters from morning and I haven’t still written all of them
I’ve been writing the letters from morning and I haven’t still written all of them/ everything

or
I’ve been writing the letters from morning and I still didn't write all of them / everything

Question

One sentence: I’ve been writing the letters from morning and I haven’t still written all of them
I’ve been writing the letters from morning and I haven’t still written all of them/ everything

or 
I’ve been writing the letters from morning and I still didn't write all of them / everything

Answer

I've been writing these letters since this morning and I still haven't written them all.