Ragman is right, of course. I'm not a therapist or anything remotely similar. What I said was just my take on what I'd think if I were in your shoes. I wouldn't drop my guard 100% with her if I knew it hurt her. She might feel that she's being taken for granted or used as a target when you vent. Not sure, of course. Best of luck to you, and again, Ragman is right about getting a pro to talk these things out with.
Edit: I just read your reply to Ragman. Scratch that last bit unless she does begin to show interest.