This began life as a reply to someone on /r/questioning, but I later came to realize that I was actually writing to myself.
(paraphrasing to anonymize)
OP expressed that they had recently begun feeling like he/him didn't quite fit, and had started talking to their non-binary friends about their feelings. Unfortunately, OP received some pushback from those friends because, they had felt NB all their life, and the process of discovery for them was mainly focussed on finding the language to express as much, so these friends began assuming that OP was faking it.
OP's main question was: Is it possible that I'm just faking how I feel? That they're right and I'm just making it up in my mind to fit in with my friends?
Labels aren't a template for behavior. They're a tool used to communicate your particular flavor of awesome to others. You are you, pick a label that fits (or don't, whatever).
There are 7 billion unique people on this earth, and we haven't developed mind-reading tech yet, so that leaves you as the only person who is physically capable of living your life, feeling your emotions, and experiencing your gender. Nobody else knows exactly what's going on in your head, and the only way we can learn is if you tell us.
That doesn't always happen perfectly, though. Sometimes the right words don't exist, sometimes the words come out wrong, sometimes both of those go exactly right, and the other party ends up misinterpreting what was said... This can affect what others understand about what you're trying to communicate, but none of that changes this one simple fact:
So, are you cis? How the fuck should I know? I don't live your life. You're cis if you say so. You're bi if you say so. You are who you say you are, because nobody else has a better source of information about you than you do.
Put another way: What if you're faking being cis? What if you were never cis from the get-go, but you've been "faking it" as your assigned gender for your whole life because that's all that you've been allowed to think until now? What if this means that, since you have no experience with a label that accurately reflects your gender identity (internal), you therefore don't know what it's like to not fake your gender expression (external)? What if it feels like or sounds like "faking it" because you've been faking an incorrect gender this whole time, and simply don't know what it feels like to truly identify with your label because it was given to you at birth based on your genitals instead of you choosing your label based on how accurately it reflects your reality?
Additionally: Human nature is to change, and neither sexuality or gender are exempt from that by any means. Knowing for certain that one label fits you for any amount of time does not mean that you're stuck with it until you die. Changing your label is just as valid as changing your haircut or your clothes. Both of these are also external expressions of internal taste in style, why would the rules for your taste in romance or sex be any different?
If your gender changes, and you decide to change your labels to match, that's it. Any resistance you receive for communicating this information to others is laughably pointless. What are they gonna do, try to persuade your experience into compliance??
Remember, You aren't responsible for how others react to information that you provide. You are simply putting it out there, and any reaction to that news is ultimately a reflection of their thoughts and emotions through the lens of their experience and bias. Your life and experience isn't affected by whether they choose to respect you by accepting information from the primary source (you) as truth, or if they choose to live in their own reality by denying it. You continue to be you, they continue to be them, and the planet continues to spin.
I realize that you may not be in a place to directly confront your friends about how they discuss sexuality and gender, but I would encourage you to push back on the idea that you're "faking" something. You are welcome to accept others' perspective, but they are as much of an authority on your gender as you are on theirs. Just because they know what it's like to experience their gender doesn't mean that they know what it's like to experience your gender.
The short version of this story is this: Your friends have shared information about their experience that they expect you to respect, but they did not to afford you the same respect when you decided to express similar information. That's rude and inconsiderate, and it's perfectly appropriate for you to
ask expect demand them to respect your gender expression (however fluid or unresolved it might be), just the same as they have asked of you.