This isn't fair to you, and it isn't fair to her. You're just prolonging the inevitable which is - this relationship is going to end. She will be all the more hurt if you continue stringing her along.

The possibility of her self harming is not your responsibility. It sounds like there's a chance you two will remain friends, and if that's the case, you can be supportive after the break up.

But by all means, don't tell her you were being fake the whole time, it was all a lie, etc. Just tell her you think you're better off friends, or something equally benign.

Learn from this mistake, and don't repeat it.