FuntomKitten, I have a friend who does that occasionally. He's a great friend, fun guy, but gets so excited and into things that he treats his old friends as afterthoughts, and it's always worse right before a convention, and he doesn't even has age as an excuse as we're both quite out of college. I was so frustrated with him last year that I nearly just cut all ties, but I wanted to give him one last chance.
Talk to your friend. But first, you need to make sure that you are calm. Talk in a neutral location, if possible (a park, another friend's house (but make that friend promise not to interfere, and hold them to it), places not your home and not her home). Tell her that you are hurt and frustrated by this. Do not blame her for things, it makes people defensive. Decide beforehand if you want to give her one more chance or not. If not, be very clear. If so, don't let her string you along if she violates whatever terms you agree on. It's hard to hold to it, but it's better for you in the long run. A good friend who did this unintentionally will try hard to at least realize when they're trivializing you. A not-good friend... they'll just keep on with it.
Though I do agree with Lithium Flower, a friend shouldn't make you miserable.