Newly diagnosed communication problems

I have some questions. I'm 24, and was just diagnosed with ADHD. I know that therapy takes time, and I'm working on it, but I had a few questions/was looking for some advice. For as long as I can remember, I have lied when I either forgot something or when I was asked about an impulsive action. I don't mean to, but I think that the constant shame I feel over constantly feeling like a disappointment to my family and my fiancee is the root cause of it. I've felt that way for as long as I can remember, and now I have a problem with impulsively lying. For example, on an impulse I made a mistake (this was pre-diagnosis), and when my fiancee found out without thinking about it I lied. She's upset (as she should be) not so much about the mistake I made (no, it wasn't an affair but for the sake of brevity I'm just calling it "my mistake") but more so about the lying. I didn't really mean to lie or to make the mistake I made, it was just like an impulsive response that I couldn't stop, and once I was in it I couldn't get myself out of it. Is this common? Do other people have the same problem? Will continuing my ADHD treatment (medication and CBT) help this, or is there maybe something else wrong that I should see another doctor about? Does anyone have any advice that's had this problem and gotten past it? Thanks