I've been best friends with her ever since 7th grade, and so far it's been over 3 years that we've laughed, smiled, and played..
But now that she's got herself a boyfriend (who has a bad reputation for dating girls: cheating, going too far, just plain rude, forgets really easily), she hardly discusses anything with me, she doesn't even care if I'm feeling lonely, down, and won't laugh or smile when I try to make her smile, she's always so gloomy and with an unfriendly attitude(which I find very annoying)

Today I tried asking her why she doesn't want to go home, she told me "she just didn't want to," it's getting to the point where she doesn't trust me anymore, it seems..

I really don't want this long friendship to end, nor do I think I can take it anymore.

When I allowed her to write on my hand (an odd habit she enjoys doing: writing on people's arms/hands), she wrote, "die ," I asked her why she would write something so rude, instead she just ignored me all the way through.

I'm thinking about talking with her, or just ignoring her.. but when I ignore her and let her have her own fun with her boyfriend, she doesn't bother to see or notice me feeling left out.

What should I do? Can I keep this friendship up?
And when I'm planning to talk with her.. I'm not sure what to say.. or even if it's worth saying anything

Well this is simple ;

Have you tried to talk with her just making a general convosation and she has ignored you and still continues to leave you out ? Well in this case leave her to whatever she is doing and act like she isn't interesting you even though you are feeling left out and annoyed. Its certain that things will go wrong with her and her boyfriend and she will then come running back to you when it all goes wrong. If you want to keep friendship with her when she does come running back to you be a friend and also explain to her how she made you feel and how she could have lost you , make it also clear that if she had lost you how could she have turned to for comfort. In the mean time have fun with other friends take pictures , go places , laugh about stuff in front of her and that will cause her to be very jelous and realise that you can fun without her and your not bothered if she is going off with her boyfriend GOOD LUCK and I hope this helped let me know how you get on :)