Agree with many points made by the posters above.
I'd say
1. If she's decided in her mind already that she doesn't trust you, it's nearly impossible that you'll be able to weasel your way in to her trust zone. The more you do, the higher she will build her wall. But! see point 2.
2. I find it an excellent sign that she told you that your comment hurt her. I wouldn't bother to create conflict with an acquaintance by bringing up something unpleasant if I believed that I am in no way going to let the person into my trust zone. If I had already refused inside my mind to get close to a person, I would just shut up and not invest energy into unpleasantries, and I'd keep everything superficial and running smoothly. So addressing unpleasantries is in itself a way of opening up, imo anyways.
3. Quality time. With my acquaintances & friends, out of sight is out of mind. Keep in touch regularly (if she lets you, she seems very withdrawn), and hang out together in the flesh. I don't know if she initiates contact or not, but definitely do from your side, invite her to do things together, watch a movie at home or something, or go do something out there in the world.
3.1. The very act of reaching out/initiating will indicate to her "what? somebody remembers my existence????" and it will fuck up with her belief system that she is alone/an island in the world.
4. Express positive thoughts about her sometimes. It must be genuine. And even then, half the time you compliment her she will dismiss the compliment, if not to your face then for sure in her own mind. So know that if you give her 10 compliments, only 2-3 of them will actually registed in her brain, if that! That's not your fault. Also, don't overdo it or anything. Go with the flow with this.
Your success will not depend on you exactly, but on her. If she has low self-esteem and has a pattern of keeping people at a distance, or if she is traumatised by past hurt or something, and if she's decided in her mind already that she will never open up to you, then that will be the result of all this, and there's nothing you can do about.
Whenever I've decided to keep certain friends at a distance, no matter what they did for years and years, I kept them in the place that I wanted them to be in, and I was having none of their compliments, none of their reaching out, none of their supposed love. I even had a female friend write me a love letter (friendship love, not the romantic kind) and she literally begged me to open up because she said she loved me so much. To this day I still haven't integrated that letter into my psyche, I just don't believe a word, and it went in one ear and out the other. My friend cried and cried, and I felt nothing for her other than pity. So... she will make silent decisions in her mind and there's little you can do about it. But you can always invest in her if that's important to you and she seems receptive, and see if the seed turns into something great
