What I take from Ahsoka's vision is that all of it manifested itself from her subconscious. Like, after Siege of Lothal, I think in the back of her head she knew the Sith Lord was Anakin, so she could have felt guilty and was playing all these "what ifs" in her head. What if I stayed? Was I being selfish? Is it my fault? etc etc. So I think part of her felt guilty and part of her knew (or at least thought it was possible) that the Sith Lord was Anakin. So that's what I got from that. I don't think she knew for sure, or maybe she was in denial, and the vision just confirmed what may have already been suspected by her.