Oh wow, sorry but just wow. You sounded so much like me just then.

I went to the hairdressers just recently, I had the exact man cut I wanted in mind. I was really excited about it. But when it came down to it Mum said my hair would look terrible short and I became afraid of what my new classmated would make of me. So I only cut it cut a little shorter and after everone said I looked cute. I felt sick, cute was DEFINATELY not the look I had in mind. My mother would have pulled a fit if I had done otherwise, but now I think it would have been worth it to have short hair for a little while. And I KNOW guys can have longer hair but I very much associate a short cut with manliness. And since I have huge bewbs I really like to feel as male as I possibley can. If it gets to the point where you desperately want to cut your hair short, you may want to concider cutting it yourself weighing up how much trouble it'll cause if you do it. It may be a wake up call to your mum to how much you actually WANT your hair cut short. It's your hair not hers, it is not fair for you to be forced to look any more feminine that you have to. Take control and be a man!
And my mum is EXACTLY the same, except she has done this the same thing before.
She never ever listens or takes me seriously and then gets angry and suprised when she has to.
I told her I liked girls, she ignored me, I said I loved my best friend, she ignored me. I said I was GOING OUT with my best friend. She found out and went balistic after I WARNED her.
And I told her I was TRANS, she ignored me, I told her again, she said that I was a WOMAN and that was the way God made me and to get on with it. Then proceeded to ignore me...
I don't know your mum enough to judge how your mum would react... BUT my mum used to hate gay people with a hatred. Now I have come up at Bisexual she still thinks it's wrong but she's more openedminded if that makes sense? She does'nt openly bash them infront of me anymore, hurrah.
Maybe when mum realises I am Trans the same thing will happen, maybe it will happen to your mum too. And trust me, my mum like NEVER changed her ways ever so it's possible for your mum too.
Again, urgh, don't know your mother at all to judge how she would take it.
My advice is possibley to test the water, maybe bring the subject up more often and ask her opinion on it. It'll probably be trash on how we are all deamons waving around points forks and etc but it's important. Your mum obviously knows you do not share her opinion of trans so maybe mention peoples stories on Susan's and how they feel trapped and their storied and etc, there are some really amazing stories on here. There are lots of trans people on sites like deviantart that has work like poetry and pictures about trangender people. (heres one
http://b00b-b-gone.deviantart.com/) If you find a particulary powerful one, when she is not busy show her one. Show her more accustomed with the idea and if there are any shows about it try and encourage her to watch it as well and maybe look stuff on wikipedia and give her facts on it. (There are some pretty cool transexual statistics) If she asks anything you can just say it's an interest, she may grow sick of it if you do it too much and yell at you and if so leave it a while before you bring it up. You don't have to do all the things i've suggested here but I think if you try something of these at least your mum will be more accustomed to the idea of transexuals and hopefully, more informed as well. Also I do think it's a good idea to have a therapist present when you have come out... Parents accept it better if a proffessional present says there is a problem.
I hope I have said something helpful.

I apologise for the HUGE blocks of text!
P.S. The suddenly IN YOUR FACE is very true for me too. Its kinda scary how I never noticed it.