Did a similar trip when I was your age. You will be fine especially since you speak the language and have flown before. A kid flying alone is going to get the attention from the flight attendants, and they will also likely offer to help you with the custom form. Just remember when in doubt, ask. Preferable someone in uniform as opposite to a random dude. Most of the random dudes are probably more confused than you.

Also bring some cash in RMB with you. Many places in China do not accept credit card.