Trump really wants to get rid of Sessions, but he hasn't screwed up the courage to do it, in the face of warnings from high-level members of his party and probably from the tame generals on his staff. Firing Sessions would be a very bad move, but I think Trump wants an AG who will fire Mueller. That move wouldn't buy Trump more than a few extra months in office and it would be a tipoff to every thinking person that Trump has something to hide. I say "every thinking person", because Trump, amazingly, still has an 85% favorable rating among people who voted for him. Most of those people will support Trump until their dying day.