After the confession was rejected by the girl? What do you mean by sending her an apology message and not returning it?

It's simple. First of all, you don't need to send an apology message. It's normal to express love. Even if it is rejected, there is no need to apologize at all. You have done nothing wrong. Don't apologize unless you hurt her when you confessed.

She won't return your messages. Most likely, she thinks there is nothing to reply to you. That's it. It's the kind of text message that can be returned or not. She can be generous and say nothing or nothing.

Another is that she is too lazy to talk to you and doesn't want to say anything to you, so she doesn't return.