People can surprise you. I was so absolutely certain that my slightly unstable, ex-marine cousin would have an issue with me and was potentially looking at some violence but he really surprised me with his reaction. He came up and hugged me and said he loved me no matter what.
It seems to me where you're at the point or coming close to the point in your life where pushing things aside and pretending things are fine isn't ok anymore. Telling your mother is clearly very important to you and at some point you're going to need to come out to her for your own mental health.
Have you talked to anyone about it yet? In real life that is. Having an ally before talking to your parents can be very helpful. Have you spoken to a counselor about this? They can provide a bit more in depth guidance than we can on an internet forum provided you are comfortable with them.
While I did say people can surprise you, parents are a bit more difficult. You are their child. They raised you a certain way and even thinking about changing that, is hard for most parents to wrap their heads around. When it boils down to it, most parents just want what is best for their children and choosing to transition certainly makes things a lot harder.
I won't lie to you, there are quite a few trans people that never really get to the point where they're all that happy with their lives and unfortunately, media depictions of trans people usually very negative. This is all your parents have to go on and frankly its pretty scary for them.
With this in mind you need to prepare yourself for the possibility of the fact that your parents may not be ok with things right off the bat. They may say crazy things,"Oh I should never have nagged you about cutting your fingernails!" (my mother actually said that to me), but really anything your parents say or do are just them coping with the massive news you will have just told them. Remember, you have been dealing with your gender issues for however long you have been dealing with them, your parents will have just started. Denial happens and it can take a while to overcome, but give your parents time. It took my parents over 3 years to really start accepting me for who I was, but it happened eventually.
I'm not exactly sure on the details of your situation, but do you live with your parents when you're not at school? Are you financially dependent on them? If you come out while you're in college, it will provide a buffer for any fallout for some period of time, but eventually you'll have to face the music as they say, especially if you live with your parents for any period of time. The buffer also will allow your parents to begin processing.
Finally, the absolute most important things you can do are to keep positive and to have a plan with reasonable goals. Keeping positive helps with the day to day and can make the lows seem less intolerable. Having a plan and working at it helps keep your mind occupied and less likely to dwell on issues that can cause a lot of pain.
I know things seem daunting, but you can get through this and on with your life. Good luck.
