I am very impulsive and never cheated on any relationship I had in my life. ADHD can increase the risk, for sure, but people with ADHD are heterogeneous. So, if you find people with ADHD who cheated or not, it doesn't mean your partner was or was not under ADHD impulsivity. I think your ex has chances to actually have other mental health issues. She can display criteria for bipolar, borderline personality disorder, etc. All these conditions can strongly influence impulsivity and sexual behavior. But never forget that human beings are not perfect. Nobody has to have a mental health issue in order to make bad choices in life. I think you should see a psychologist because you sound like an enabler and your relationship seems very unbalanced from what you describe.