Hey,
First of all, that is great to hear! I remember the early days of first passing and it just fills you with such a great feeling! And when I first got my binder, it was great to see the work it did

(Despite the discomfort that I hate! :/ The years takes its toll on you but Im still grateful for the effect they have).
Its funny because at the beginning of my transition I was seen as a little skater guy too :p
As with your Mum, I can imagine how you may be feeling very worried. I remember before I told my family, something came on tv about a transwoman and my step Dad commented saying theyre all freaks. But that was a very uneducated and ignorant remark. He had never encountered an mtf or ftm and understood what they go through, that it is not just some whim or rash freak decision. When I told him about me, he was extremely supportive, more so than my blood related family. So you may be surprised.
Telling my Mum was very hard, we are so close. I found that it helped to let her know I wanted to tell her something, and then write a letter and give it to her. Some people say that isnt good, or its the cowards way, but I think it is a pretty suitable way. You can get all of your feelings and explainations written down, with no interruptions. They have time to read, digest and take it all in and think of how they feel and how to respond. Sometimes people need to time get over their irrational initial feelings and process everything.
I hope everything goes well for you and that you get even more passing experiences!