People sometimes can't understand why they don't understand the Bible. Well, the way to understand the Bible is to obey the Bible. And the way to understand the verses you don't understand is to obey the verses you do understand. 

"More than the ancients" is a way of saying the accumulative wisdom of the ages. David is saying, " I haven't been off to the university perhaps, or I haven't studied abroad, but I keep God's Word and therefore God reveals to me His truth."

The only way you can really understand the Bible is for God to reveal it to you, and God doesn't reveal it to rebels.