Pressing On, I Forget the Past

Phil 3:12-13

12 Not that I have already obtained it or have already become perfect, but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus. 13 Brethren, I do not regard myself as having laid hold of it yet; but one thing I do: forgetting what lies behind and reaching forward to what lies ahead,
NASU

Question: What is the one thing I do?