Not that I have already attained, or am already perfected; but I press on, that I may lay hold of that for which Christ Jesus has also laid hold of me. Brethren, I do not count myself to have apprehended; but one thing I do, forgetting those things which are behind and reaching forward to those things which are ahead, I press toward the goal for the prize of the upward call of God in Christ Jesus.
Philippians 3:12-14 NKJV
There a couple different ways that we can understand this passage. (I'm sure there's more, but I'm going to focus on two.) Some of us have a past that we don't like. It could be experiences; things we did or were done to us. It could be who we were back then, not serving Christ and doing a good job of it.
There could also be others of us who look back and consider ourselves to be pretty good people. Raised in church, saved at 3 years old, etc. This is the boat Paul, as Saul, was in. He was a Hebrew of Hebrews. That means he didn't "defile himself with any of the ways of the Gentiles who had take over his people."
He knew the pentateuch inside and out, and was a descendant of the first king of Israel. That's like saying someone was born on the steps of the White House to two American parents waving an American flag while an eagle flew overhead reciting the pledge of allegiance. Can't get much more 'Merican than that.
After Paul's experience with Jesus on the side of the road (Acts 9:1-22), all of the sudden all of that meant nothing. Not all of it was necessarily bad, but it definitely wasn't the way to be accepted into the Kingdom of God. Then there was the stuff he did that was just plain awful that he can never take back. He could have dwelt on who he was, hoping to minimize the bad and work to maximize the good, but he recognized that he didn't need to. His justification (and ours) comes solely through our faith that Christ died for us and redeemed us. What did Paul do after this realization? He decided to leave the past, good and bad, in the past and move forward. Making his new goal to be Christ-like. His prize, the call of God on his life to further the Kingdom for generations to come. The law helped him know of God. His experience and subsequent relationship with Jesus helped him know God. His salvation? Through the blood of Christ and nothing more.
I think of this verse when I'm driving. I have a horrible habit of looking in the rear view mirror for extended periods of time. It feels like Holy Spirit speaks to me and says "focus on the road ahead." The rear view mirror is an excellent tool, but should never be the only place I look. I could get in an accident if I am too focused on what's behind, rather than what's ahead. If you have accepted Christ as your savior, that past is behind you. It's okay to let go and forgive yourself. Look to Jesus and press on to what's next.