You really can tie yourself up in knots of worry, sweetie. Just calm down, all you need to think about is getting to the appointment.
Write down a few (and only a few things) you absolutely want to say and have it discreetly with you on the day in case you get lost or tongue tied. I would suggest you speak to the heart of your gender identity issues and speak honestly about how you feel in the moment.
Avoid going into fabricated realities and fantasies ("if only I was a girl everything would be awesome"). Acknowledge that you understand that transition is extremely difficult, that it will be challenging and that you have a few other personal issues to deal with (your mom, BPD) but with the right support you believe you can do it.
Have a realistic sense of what lies ahead, don't get distracted by side issues... etc. but most important, be calm...there's nothing wrong with being emotional, they need to understand that you have deep feelings about this personal issue but if you break down in tears every sentence you won't get far.
You won't be able to tell them everything, and that's not the point...just focus on how and why you don't feel your assigned gender is your actual gender. Be calm, be centered, breath...smile, you'll be fine.
