Why would she back off, ater telling me she likes me?

You answered this question yourself Fred. She isn't over her EX and unless she's a total ho, doesn't want 'a friend' in her life right now. Problem is you want more than friendship, and she doesn't. Why not try to be a friend, back off and tell her you understand where she is coming from, you feel her pain (and she is in pain) and you are there for her if she needs a shoulder to have a cry on. Back off, be a real friend and maybe, in time there may be more, but not now Fred, not now.