How do I know my understanding of Scripture comes from the Holy Spirit and not my own interpretation?
Correctly understanding the truth of God's word is never a matter of personal opinion or private interpretation, as Peter says:
Peter reminds us that we have been given the word of God, which is a sure and reliable thing. God has the power not only to deliver His word to us but to also ensure we gain the proper understanding of it. In v.20 Peter says that the correct interpretation of scripture is never found in one's own opinion. In other words, we do not arrive at the truth of God's word through assumptions, opinions or "feelings." No different than the way one arrives at a proper understanding of mathematics or biology, we must study scripture with an expectation that the truth is objective and absolute. It does not bend to the will of men.
On the other hand, scripture also teaches us that our ability to uncover the correct interpretation of scripture is a God-given ability. Notice in v.21 Peter says that all scripture was authored as a result of the Holy Spirit moving a man to record the words of God. Therefore, since the word of God was authored by the Spirit, only the Spirit Himself can properly interpret its meaning. This is why Paul says:
All believers have been given the indwelling of the Holy Spirit so He can teach us the true meaning of scripture. In a sense, the Author of scripture explains its meaning to us as we endeavor to learn it. If men absent the Holy Spirit endeavor to learn its meaning, they will inevitably come away with a false, distorted view of scripture, since they lack the Spirit to guide them and reveal the truth.
More over, believers will misinterpret scripture from time to time, but unlike unbelievers errors, these misunderstandings are merely the consequence of the normal process of moving from spiritual immaturity to the fullness of Christ:
In other words, a healthy Christian committed to maturing in God's word will steadily increase in the correct knowledge of God's word. Whatever mistaken understanding we may acquire in the early years of our studies may eventually be corrected by the Spirit, so long as we persevere in our studies and maintain a teachable heart. People who become stubbornly unteachable will cease maturing and may be left with many false views of scripture, since they have stopped listening to the Spirit.
The desire to have Jesus explain everything all at once is understandable and common, but it is also potentially harmful. The Lord expects us to devote ourselves to a lifelong study of His word, for this is the way we maintain a close, daily relationship with Him. Were He to explain everything in His word to us all at once (assuming we were even capable of absorbing it all), we would have no incentive to maintain a daily relationship through His word.
Our spiritual goal isn't to understand the Bible; our spiritual goal is to know the Lord and grow in the grace and knowledge of Jesus Christ through a study of His word.