Hi William, and welcome!
I hear what you're saying about it being 'only a phase'. Those are the exact words my parents used to invalidate me, and they were so convinced that I was going through a phase that they actually became angrier and more annoyed as time passed on and I didn't grow out of it.
I came out to my folks when I was five. I told them that I was really a boy, that I wanted to be known by a boy's name, and that I wanted to wear boy's clothes. My mother told me that I was just going through a tomboy phase and that I'd eventually grow out of it. The prevailing wisdom at the time was that as soon as puberty hit and the hormones started doing their job, I would grow out of my tomboy phase and I'd settle down to being a girl. And you know what? That advice is 100% accurate for most tomboys, because they are genuinely going through phases.
The problem was: I wasn't a tomboy. I had (and have) a deep-seated discomfort around girls, mainly because their behaviour and interests were completely and utterly alien to me. I might as well have come from a different planet (Mars, I think, if that book is correct, LOL). As soon as I realised that I was not anything like the other girls around me, I realised that there was something wrong. But I was only 5 years old; how many 5-year-olds have heard of 'transsexualism'? And how many parents, particularly back in 1976, had ever heard of a 5-year-old girl saying she's really a boy... and actually being right?
So I suffered extreme discomfort whilst hoping that my mother would be right and that puberty would make these horrible feelings go away. But of course, it didn't: it made them much, much worse. I did indeed go through phases: I tried dresses, make-up, long hair, a Goth phase - all as ways of trying to find self-acceptance as a female. But whilst to me those were temporary phases, every time I tried something like that, my mother in particular would take them as evidence of me growing out of my tomboy phase. *Sigh*. I couldn't win.
And since then, I've never felt comfortable with being female. I have had long-standing, persistent discomfort with my gender. And that, my friend, is pretty much the first and most significant criterion for a diagnosis of 'Gender Dysphoria'.
So, what to do? Well, you haven't yet mentioned here how long you've been feeling this way (and nobody is suggesting you should unless you feel comfortable to do so), but the truth for many trans* people is that we learn to hide the way we feel, so that other people don't notice we're different. We do this to avoid being criticised or bullied. But an unfortunate consequence of this is that the people around us have no idea that we're suffering, or how long we've been suffering. So when you finally pluck up the courage, after many years, to tell them that you're uncomfortable with your gender, to them it can seem like it's a new thing because they haven't heard of it before... because you've been hiding it from them in order to protect yourself.
So my advice to you would be to think about how long you've been feeling this way. If it's been many years, tell this to the people who keep saying 'why' or 'it's just a phase'. Help them understand that you've been feeling this way for X number of months/years/whatever, but you just didn't feel safe to tell them earlier because you were frightened of being punished/ridiculed/whatever.
I found the best way to convince others of the truth of my gender is to live it, every single day. People believe the evidence in front of their own eyes far more readily than they believe what you're telling them, so make sure you live every aspect of your life in a role that is appropriate to you. But remember that your friends & family will always be two steps behind you because they haven't lived your life, so you'll have to be patient and wait for them to catch up with you.
Transition is a marathon, not a sprint. It can take several years to get the therapy, hormones, surgery etc. to put right what nature put wrong. But it can also take us many years to find the courage (and the funds!) to get started on the medical side of our journeys, too - and we're no less trans* whilst we have to wait to get started. Patience is not just a virtue, it's an essential requirement for any transperson considering or undergoing transition.
Good luck.
