I sent several messages to the girl I secretly love, but she didn't reply to me. I really want to ask why. How can I ask her politely?

There are two reasons for not replying. First, she is not interested in you, so it is difficult to refuse. Second, it's interesting to you, but you are too euphemistic, uncertain and don't know how to reply. So, the only way is direct. Anyway, just send a text message to say p euphemistically, you will know sooner or later. You have to let her know if you are polite. It's no big deal to be rejected, but if others are interested in you, you have to hit the wall.