Above all else, you need to be honest with her, and also find as many resources as you can to help her learn what this is about ... and then let the chips fall where they may. You are both young and if it doesn't work out it is best to find out now than in 10, 20, or 30 years from now. Or, she could surprise you and your relationship might be strengthened. But, you can't live a life of repression, nor can you "make" her accept anything that she believes is fundamentally wrong.
Most colleges have LBGT groups and I'm sure they'll have excellent resources.
Another thing to consider: there is immense freedom in the college environment. You can go to class in a skirt and pretty top, and as you say, most people will think it's OK. But, this is not the case in the working world, where a bulk of the people are a great deal older than you. Not only do most employers not take kindly to the CDing, they don't even like to know that their employees are CDers ... unless you're headed towards a creative and liberal line of work in a geographic area that is equally liberal.
So in your conversations with your girlfriend, I'd take this into account. You might want to discuss who you both know that she might feel comfortable telling, for example her family, your family, the members of your religious community, her childhood friends and yours, and anyone else in your lives. In other words, discuss the degree of "outedness". It's one thing to find acceptance from people at college who are there for a short time just like you are and who will move on, and another matter with people who are closer to you both than arm's length and also prospective employers.
I wish you all the best with all of this and I'm not saying the CDing will not work out in your life, or that your girlfriend will not be OK with it. But, you do need to consider more aspects than just wearing the clothes to class.![]()