Don't worry too much. Self-harm is really common with PTSD, so I don't think she will be too surprised by it. Not admitting to things is also, I'm pretty sure the absolute norm. The things you are ashamed of or run from the most, are the very things you will keep private until a certain level of trust has been built. Another thing commonly done by people with attachment disorders is to push away once vunerable...so everything you are doing is normal and honestly expected. This likely, is not at all her first rodeo. I've had a few things that I planned to admit and were really, really hard. Sometimes it took several attempts to get it out. And once mentioned, I have refused to go back and touch and she hasn't pushed me to go there. I just needed her to know that these things were there and happened.
As far as self-harm goes, my therapist asked me about it on our first session, I lied, she called me on the lie and then said she knew I self-harm by the way I was playing with the hairband on my wrist. wtf?! I was literally froze. In my head I was like, "Oh shit, what the hell else do you know that I have not told you? I'm not sure I'm comfortable with this." The answer: she knows A LOT. I never attempted to lie again though--I just refuse to respond when I don't like the question. lol.