Paul had been a well-educated Jewish leader before he trusted Christ. He points out that if someone has reason to be proud of his accomplishments, it would be him. He realizes none of it counts for anything when compared to his relationship with Jesus. He tells them to forget their past mistakes and accomplishments and strive to be the best God would have them to be.
It’s hard not to dwell on our past. Sometimes it’s guilt over a past mistake or pride over a success. Our best option is to look at where we are going and work hard to get there in the best possible way.