Can I regret on a decision of filing for custody and cancel the whole application? What if I dont show up in court?

If you want to withdraw your motion, you should call the court and follow-up with a letter advising that you are withdrawing the motion. If you don't withdraw it, you run the risk of having the opposing side file a cross-motion.

You can withdraw the application or not show up, either way the matter will be dismissed.

You are welcome to withdraw your application. Contact the Court and let them know that you no longer wish to proceed with the application you filed yesterday. They may require (and most likely will) that you provide something in writing to them. Best of luck.