I told my mother quite some time before I even started seeing a therapist, and I was also sure to get her support. And I told the rest of my family around the time I started seeing a therapist. This was almost a year, if not more then a year before I started dressing in womens clothes. And looking back it has worked out really great for me, as it has given my family time to get used to the thought over time, even before I started transitioning for real. Mind you I'm pretty sure they did not understand it at all or get that I was serious until I did start to transtition for real and started to occationally wear womens clothes. Because I could still see the shock in their eyes when I went out to dinner with my parents wearing a lovely dress for the first time. But atleast they knew why, even though it was a shock to see their oldest son becoming a women and presenting as such.
I would say unless you're sure of bad consequenses for telling your mother, go for it. I've been suprised at how open and supportive 95% of the people I've told have been. It's amazing how the people around you that really care for you, really do care for you, no matter how you dress, present, or what gender you are. My own mother is amazing and have a very simple view on things (well it's very simplified, but it sounds great to tell it.

); "As long as you're happy, I'm happy.", and when it comes to clothes; "You can wear whatever you want, as long as it looks good."
These are some of my experiences and again, I would say, go for it.

You might be suprised how supportive she'll be.