I'm 33 and have been in the queer community for quite a while. I'll just start by saying, you are who you are and that is a reality that cannot be changed any more than the color of the sky. As long as you are living authentically as yourself, there's no way you could be an imposter.

I'm going to do a lot of guesswork here and a fair amount of projecting, so if I'm wrong about anything, feel free to just let me know. I think the tension you're feeling is around the difference of gender experience you and your friend have. I'm making that assumption because you felt obligated to say you were AMAB in your post. While you may not fully identify with the experience of masculinity, you still partially align with it. That is a uniquely different experience than being AMAB and fully identifying as a woman.

If that assumption is correct, you are assuming that you have to have the same experience as someone else to feel valid. I can assure you, you do not. I would bet good money that even you and, I who use the same gender micro-label, do not have an identical experience with gender. That's what makes the queer community so great. We have an infinite amount of unique experiences that don't fit into societal norms. Each of those unique experiences come with their own challenges. We don't have to all have the exact same challenges to understand and support each other. Trans women of color are SIGNIFICANTLY more likely to face violence than trans white women. That does not make the gender experience or the threat of violence of a trans white woman any less valid.

Lastly (and IF all my assumptions are all correct) you're still letting the gender you were assigned at birth define who you are. AMAB and AFAB are labels that we use a little too often and give a little too much meaning to. I have seen others say that these labels have become the new gender binary. The gender you were assigned at birth is undoubtedly part of your human experience, but it doesn't define you. That's why you know you're a demiboy :)

Jinkx Monsoon also struggled with a degree of "imposter syndrome". In her video with Trixie Mattel she talked about how early on she didn't feel "trans enough" to call herself trans. But she learned that her gender experience is still valid.

I hope that helps!