At 18 my attention span lasted about 8 hours on anything. Which meant if I couldn;t get "it" done I had a hard time with "my failure" and would struggle to finish larger projects. My advice - have some patience, and recognize the tiny bits that make up the whole. You won;t sand it all in a day. You won't fix every dent in a hour, you may have to take things apart to fix something tiny the right way. Just do it - whatever time it takes. You're 18. You'll have plenty of time to make it what you want.