I have been praying salatul layl since Shahr Ramadhan and I happened to have missed only one, I feel really bad and sad, any to offer it in Qadha? Or how to catch up on the one I missed?

Yes you can pray it Qadhaa. It’s recommended to pray it Qadha during the day, not at night. One hadith states the Qadha of the Nafilah has a greater reward because it shows how the servant of Allah is willing to make up something that wasn’t obligatory in the first place.