I’m insecure about my friend-please help?

Have you tried talking to her about it? Or maybe asking her what has upset her? That would work well if you could understand what happened. Then move on from there, if it's something you can fix and the friendship is worth it then try to salvage it, but if it's something you can't help or something that's just you, perhaps talking to her about why you do that or are that way would be the most beneficial?

You can apologize for whatever set her off and try to address the issue and find a common ground between you two. Generally, you might want to give her some space and air to breath since it seems like it's what she wants at the moment. If you try to talk to confront her too much or in the wrong manner, it might pester her instead of helping keep her as your friend. Because she's been your friend for four years, the space might actually bring you closer.

She's the only one who knows how you can be her friend again, so ask her. Be polite, and ask her something like "What am I doing to stress you out?" Then, consider her answer. If it's something you can change, like remembering to text her ahead of time if you can't meet her somewhere, then do it. But if it's something that's a bigger part of your personality, then that's kind of the end of it, sorry to say. (You can't be friends with a person who doesn't want to be friends.) But if she says she still respects you, you should take her at her word. Don't assume she hates you if all she says is that she gets stressed out around you, especially if you've been friends for years.

I am just wondering, do you actually know what you did, or remotely understand how/why she could feel uncomfortable around you?
If you have no idea what you have done wrong, then as 'Anybody' said, you should ask her. To understand and have some kind of closure.
But if you kinda know what happened or was happening, then you are the only one who can judge if anything may be done at this point.

A couple years back, I discovered a friend's private blog and I told her I did. She felt violated and never talked to me again. She was scared to death of stalkers and I knew it. I didn't mean to do what I did and didn't expect her reaction.
I screwed up, still regret this, but I know nothing can undo that.

Thanks so much for your answers, everybody! :) In response to TheWarlord, I'm not really sure exactly what I've done wrong. I have ideas, but I'm not sure. I plan on asking if she's willing to talk, and seeing what happens from there.