It may not go over like a ton of lead balloons. It will likely be a shock, yes, but she may suspect something anyway, and is waiting for a confirmation without asking you outright.
Not an easy thing for a parent to deal with, and difficult for you to tell her. You might consider the guidance department of your school, or a local support group for other young gay people if there is one, for information. Do a little research on line, and consider talking to a counsellor who can offer some advice.
May be a good idea to have another person with you when you do tell her. That will help to calm the waters, and you'll have the support you need, and so will she. It is not the end of the world.
I'm wondering too that all the difficulty you've had together over the past several months, hasn't had to do with this somehow. Maybe this will turn the corner; by being mature and honest about who you are. That will open up conversation and communication hopefully, and the road together won't be so difficult.
It won't get any easier with time, and likely will be more stressful for you, which you may be taking out on her.
My advice is to be as informed as you can, have somebody with you, and just tell her.