The thing is, people very often have selective memory. So to us, it's very obvious looking back that we were gender-nonconforming and that those experiences are a validation to us, because those memories are the ones which are important to us. Where to our parents, they often have their own fixed narrative for us in their heads, and only choose to see or remember the elements of our pasts that fit that narrative.
And, well, one doesn't even need to be gender nonconforming in order to be trans. It's a symptom, not the problem itself. So in my view, focusing on the gender-nonconformity will only allow her more space to say "yeah, I know you did these few things that were indeed gener-nonconforming, but you also did this and this and this that wasn't gender-nonconforming, therefore doesn't that prove that you're not trans?" I'd recommend focusing on the long-held sense of self-identity instead. Rather than talking about how you acted boyish as a validation of your male identity, talk about the male identity itself, which has presumably been long-held and unchanging regardless of what you were doing on the outside. That is something that can't be mentally invalidated with her reasoning it away, because you're the only one with access to your own core sense of self. When you tell someone that your identity is that of the opposite sex, and that it's been that way for a long time despite what you were doing on the outside, the only answer in response to it is either denial or acceptance, with no room for saying "no, you're wrong, you didn't do that, THIS is what you actually did."
I'm not sure what will work and what won't, because to a lot of people gender, sex, and gender expression are so tightly-knit in their heads that they can't imagine one without the other. But I think it's worth at least trying to tell her that even if you did do feminine things occasionally, that doesn't disprove your identity, because gender identity is a completely separate entity from gender expression... like, gay guys often do feminine things and act in very stereotypically-effeminate manners, but does that mean they're not male? Lots of young girls are extremely gender-nonconforming to the point that they won't do anything feminine, but does that mean they're not female? According to the APA's studies, even kids who are so gender-nonconforming that they're admitted to a clinical psychologist, are much more likely to be cis than trans. And it completely accepts that some kids who expressed no gender-nonconformity whatsoever can still be trans.
I'm not really sure how to phrase this so that she might be willing to question her views on what makes someone male or female, but there's got to be a thought experiment in there somewhere that will help her learn that a gender identity is something that is completely separate and valid regardless of behavior and how others perceive them.
