There are a few things here that should be very reassuring to you: One, she told you. If she wanted to be able to hide it from you, she wouldn't have told you that she used to self-harm. Two, she hasn't self-harmed for 18 months. That's a very long time to go without, and probably means she won't go back. I know that it's hard to stop thinking about it, but at least try to reassure yourself. She's not hiding it from you, and she hasn't done it for a long time.
The best way for her to become less self-conscious is for you to ignore them completely. Don't avoid or pay any special attention to them. If you're touching her arm and you run accross a scar, keep going. It might take a little while, but if you don't notice them, she'll forget about them, too.
Okay, now this is the hard part. You cannot protect her. You cannot be a buffer between her and the pressure she feels in any area of her life. You can help, but you can't be her only resource if she needs help not self-harming. Don't take on that responsibility. I'm having a hard time wording this, but basically, you can't be the only one she leans on, or expect to be able to fix everything for her. She has to deal with the world around her. The more responsibility you take on, the more strain it will put on you, and the more strain on you, the less you can help her. I hope that makes sense. Also, in the event that she ever would self-harm, even if it's just one time, you can't think of it as a failure on your part, or hers. Roasted Carrots touched on this. You seem to be taking this really hard. You need to talk to someone about it. My suggestion would be to talk to someone else first, and then, if you feel it's something you want to talk about with her, talk to her about it. She may be able to reassure you on topics like starting again, and you could probably state that (assuming it's true) you'd want her to tell you if she felt like it, or already had, so that the two of you could work through it. Just make sure that more than the two of you are in on this, because the more people around to lean on, the easier it will be to find support whenever you need it, and for whatever reason.
I don't think that you've been overly negative, or excessively worried. I do think that you need to talk about this to someone who can talk you through it. (Excuse my redundancy, it's getting late for me.) Whether it's a counselor at school, a trusted friend, or people on here.
