Well, here's the story.

I've been best friends with a very special person in my life for 10 years, going on 11.
But, 2-3 years ago, she started showing signs of depression.
Like any good friend, I was there for her. I let her come over, and talk to me whenever she needed to.
Time passed, and she got worse.
She started cutting, and hurting herself. I literally took some of the knifes out of her room and hid them in my own. After maybe a few months of cutting, she over-dosed and nearly killed herself.
She was sent to a hospital for 2 weeks, claiming that she could hear voices in her head, and someone made her do it. I stuck by her the whole time. Even though, much of this cause way too much stress for someone my age.
After this, she still wasn't better. More cutting, and at times drinking. Some nights she would call me saying how un-safe she felt. That if someone wasn't at her house she might hurt herself.

A large part of me believed she just wanted attention, because she never actually tried to kill herself after the first attempt. But I didn't want to believe that my best friend was using me.

Day after day she does this to me, says she feels weird, or something's wrong. I'll drop whatever it is I'm doing and help her, but in the end, nothing was wrong.

I would love to say no, to tell her I don't want to come over anymore. Because now, she is better. She hasn't cut in months, or had suicidal thoughts.
But, if I do say no, she starts to act apathetic, or dry towards me. Or makes me feel insanely bad in general. And I don't think that's fair.
She knows I'm not a person to say no to someone if they come up to me in need.

So...that's partially why I think part of her may be using me. My parents are always telling me how they are so worried about my emotional strength, that she dumps all her problems on me. But I'm also the type of person to hold in my emotions, and help others. But in the end, it doesn't make me feel better.
I feel worse...

And I don't want to be the person who always does that.
Sure, I like helping people, but at times, if it's affecting me too much like this, I can't do it...

I just don't know how to approach this...
Advice?

What your friend went through/is going through is tough and there's nothing wrong with being supportive and helpful. But since she's doing well, she does have to learn to cope without you. It's unrealistic of her to expect someone else to drop their life to cater to her needs, no matter how close of a friend you are. You just need to sit her down and tell her that you are her friend and you are glad she's doing better, but you cannot drop everything to be with her all of the time. It's unhealthy for you to have to do that and hold in your emotions to be there for her. If she's really your true friend, she'll try and understand. If not, you'll know that she was/is just using you. But it's ultimately up to you to talk to her.

I've had somewhat of the same problem, though my "friend" literately punched me.
I know it is hard, believe me, but the best thing for you would be to cut all connections to her, because if she uses you like that, she's not gonna miss you, she doesn't even care much about you now, even if she might say so.
And don't listen if she is saying she will commit suicide if she needs you, it's only a way to keep the grasp on you.
I know it's hard to lose a really good friend, but think about it this way: Is she really a friend if she's just using you?

You have to think about yourself, because at some point you'll break, and you might need even more help than I did...

I completely agree with both of them. You need to give her space, she has to realize she is not a baby anymore, show her the strength she really has inside of her. If you do that, then you are the best friend anyone could wish for.