I feel bad for what I did to my female student?

I have a female senior student (1😍 in my class, which I already do have for three years now. She used to be pretty optimistic and got good grades too. Then this year she changed : She doesn't pay attention in class no more, writes bad grades and that she doesn't give a damn for my class anymore. This really frustrated me that's why I didn't pay much attention to her anymore.

There is now this other student who is a classmate of hers and is really motivated, smart and participates in my class which I admire. I give her the most attention because I really see that she is interested in what I am teaching. Today I taught in their class again and everything was like ever : The first girl didn't pay attention and was lazy and the second one was very hard-working. After class, I heard how the first girl said that it wouldn't be even worth it anymore to do anything in my class because I'd thought she is dumb and that I would prefer my "favorite" anyway. I was kinda confused and immediately confronted her about it. She said that she felt like I thought she was dumb and so she just wanted to work hard for classes in which teachers believed in her. I felt so bad at that moment because I didn't realize how my behavior hurt and affected her emotionally. I then apologized and she just shrug her shoulder and said
"Nevermind. You aren't the first teacher thinking I'm dumb." and then left my class...

After that comment I started to feel really bad and I don't know what to say. I feel so sorry for her but also guilty... What should I do?
I feel bad for what I did to my female student?
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