Matter of love and death

This is kind of a lengthy situation. Please post any advice on what you can.

I am 16 years old. My girlfriend, Mary, is 14.

I have been dating Mary for about 8 months. I want to break up with her, but I am very scared to. Not because I won't because to deal with the pain, but because I don't think she will. She is emotionally fragile, and takes extensive therapy and medication to cope with her depression. Along with that, we have both suffered through our friend's suicide within the past year. She is still fighting through the emotional effects of that. She also has a terrible home life, involving parents that fight all the time, and a 20 year old college-dropout brother that belittles her actions and insults her at any given moment, which she takes very very personally. I am afraid that if I break up with her, she will hurt herself, and potentially kill herself. She has attempted suicide, and self harmed before we dated.

I have told her many times during out relationship that even though we might not date forever, I will always be there as a close friend to her, to be there for her whenever she is sad, or depressed, or needs to vent her emotions out about anything. And I've meant every word of it.

But wait. There's more.

Within the past few days, I've gotten very close to my friend Jill. She is 18, and heading off to college in a matter of weeks. across the state (five hour drive). We've talked a lot, and confided with each other over the phone, and even helped each other out with our personal problems. (I even talked her out of self-harming). We've met each other at the mall, and within a few more days, we were kissing.

Recently, Jill's mother caught us kissing, and is now accusing the situation of being criminal, due to me being underage. She is now grounded from seeing any of her friends, until she leaves for college.

My question is.... How do I break up with Mary, and how do I deal with the situation with Jill?