How should a Christian view their "past life" (life before Jesus)? Paul says to "forget what lies behind", however we are also told in the Bible to remember what God has done for us, and to remember both the good and the bad we have lived through. Taking in context, we see that Paul is saying that we should forget the trappings of how we used to live and to strive to live this new life with Jesus.
The Lord may reveal what your life ministry will be through your past when you're strong enough to face those experiences. He is not simply calling us out of sin (for which we are forgiven) but prompting us towards a goal (we are not to be complacent). Not every goal satisfies; we are created to have a relationship with God and without that we will always feel our lives are lacking. Jesus is infinitely fulfilling.
Don't give up, your future is before you. Even if you stumble, lean on Jesus as he encourages us to press on. Glorify the one who has given a second chance.