I’d go in and say that you started self harming again. That it used to happen in high school, but you didn’t talk about it with her because it wasn’t something that was bothering you up until now. If you feel like you should mention the one time you lied, you can mention it happened once while you were seeing her without going into specifics. Lots of people take some time before sharing such things, I’m sure your T is used to that.