I'm sorry you find yourself in this difficult situation. You didn't mention if you're receiving mental health services. (Perhaps you've talked about that in previous posts?) I'm not a mental health professional. So all I can offer is my personal opinion. You mentioned pathological / compulsive lying. Here are links to 3 "Ask the Therapist" columns where people wrote in to Psych Central regarding their own compulsive lying along with the therapists' responses:

Am I a Compulsive Liar? - Ask the Therapist

Why Am I a Compulsive Liar? - Ask the Therapist

Compulsive Lying and I Can't Stop - Ask the Therapist

You also mentioned believing you may have ADHD but your girlfriend is having nothing of it. I think, first of all, the best way to find out whether or not you actually do have ADHD is to get tested. Then you'll know & can get appropriate treatment if you do. But, beyond that, perhaps you need to think about your relationship with this young woman. Spending your life with someone who is willing to reject out-of-hand, so to speak, your concern over the possibility you might have ADHD (or whatever the condition might be) doesn't, to my way of thinking, bode well for your future together. I'll tell you that, in my case, I've waged a life-long struggle with my gender identity. But my wife simply doesn't want to know anything about it. So I keep it all to myself. And let me tell you... it's tough. But that's the way it is. I can't recommend it though. Best wishes...