I don't know how to cut ties...

I have a friend who is a little intense for me. We met in school about 6 years ago. She ended up dropping out and we haven't talked for a while, but we met up again like 6 months ago.

Before she did she told me some stuff I would have preferred not to know. I wont repeat it cause it's not my story to tell. Suffice to say, I can deal with a lot of crazy shit, and this is too much for me.

She's been going through some really bad times, been with the wrong guys, self harm, depression, anxiety, pain disorders... and it's just too much for me. I feel like such a horrible person but I can't help her. It's to emotionally taxing just to be around her. And to add to things, she had the wrong idea regarding our relationship in school. She messaged me about 4 months ago and said something to the effect of "I know you're in a long term relationship with you boyfriend, but you know, we could still be friends with benefits." I don't know where this came from. I am 100% straight (I have NOTHING against gay people or any other variation).

So between basically hitting on me, the thing she told me years ago that, try as I might, still scews my view of her a little bit, and all the problems... I just feel like it's in my best interest to cut ties. I can't be that person for her. I have enough trouble keeping my own life together, I can't help her. I hate that I can't help her but keeping her in my life will do more damage than good.

But I'm aware she has all these mental/emotional/physical problems... I don't want to cut ties with her in a way that could damage her further. I don't want to do any damage to me or her. I've tried the cowards way out by just not replying her to messages for a couple months... but she keeps texting me "how are you?" about once a month. I don't know what to say that wouldn't completely destroy her, being so fragile already. And I kind of hate myself for not being able to be that person for her.