((((((((((Halliebeth)))))))))))
I reccomend getting rid of the glass, and then asking your friend (don't do it in a confrontational way, though) to understand how hard it is to quit si. Maybe you could even find a book or a webresource that specifically addresses how hard it is to quit. Then maybe she could understand a little better where you are coming from.
If she is not very open to talking, you could try writing a letter. Tell her how much you appreciate her support and how good to you she is, and then tell her that you don't want to hurt her by what you are doing, but it also isn't something that you can stop overnight. You need to gain the skills to be able to cope in healthier ways, and that takes some work. Maybe if she understands how dependent on cutting you feel right now, then she can wait out the storm with you.
Best of luck to you,
Angela