Psalm 119: Nun

In Psalm 119:106, we read: "I have sworn, and I will perform it, that I will keep thy righteous judgments" (KJV).

This verse puzzled me.  A person is swearing that he will keep God's word!  That's pretty serious, considering how strict God in the Hebrew Bible is about oaths and vows.  Ecclesiastes 5:4 says that it's better not to make a vow than it is to make a vow and not keep it.  Numbers 30:2 affirms that a man who makes a vow or an oath must perform what comes out of his mouth (but Numbers 30 allows for a father to nullify his daughter's vow, and a husband to nullify the vow of his wife).

Why would the Psalmist swear to keep God's word, when there was a good chance that he (like all of us) would fall short of keeping God's commandments?  Isn't that kind of an oath unrealistic?