I think honesty is the most important component, let her know it's not a phase or a fad and that its what you truly want and need to do, that to not live your life as your true inner gender has hurt you and not allowed you to have a full and proper relationship with her, that's what I told my mum and it wasn't easy or painfree, it was a very painful and upsetting conversation but I was completely honest about what I was going to do, some of which I have already done (change my name, live full time) and some I have yet to do (hormones, surgery) so that's my advice, don't dress it up in fancy terms, don't offer false hope that it may be a phase if you know its not, just be open and honest and I think your mum will thank you for that much at least, even if its not what she might want to hear
