Upset over Facebook :(?

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I left school in June, but before I left, me and my teacher became really close (we're both female). She told me she wanted to stay in touch after I left and told me she had Facebook. So after I got my GCSE results, I sent her a friend request, so if she accepted I could then inbox her and tell her the grade I got. She denied my request and I now feel really upset. If she would've sent me an inbox saying she couldn't accept me due to her privacy, I would've completely understood and been totally fine with it, but she just denied it and said nothing. I now feel really bad for sending her the request, eventhough it was her who brought Facebook up and suggested staying in touch in the first place. I feel like I've offended her and she doesn't likes me anymore (as she didn't just ignore it, she denied it). I just want to tell her I'm sorry and didn't mean to offend her, but I can't inbox her and I have no way of contacting her. I've seen her since I left school and she was really nice to me, so I now feel really bad. I know I shouldn't take it to heart and I shouldn't take it personally, but I do and she always trusted me and said how I was a lovely, mature person. So what should I do?
She's left the school and moved away so I can't go and visit her. She told me she wanted to stay in touch and I agreed I wanted to too, so I can't understand why. I only wanted to thank her for all of her help.