Yup. About 3 times in my poker career I have struggled to get past an arbitrary but obvious "target" - £125, £400 and £900. The only thing you can do is forget about the $600 mark - it IS only arbitrary, after all, just like $152 and $671 are arbitrary, and concentrate hard on playing solid poker. Ignore your bankroll (assuming you don't lose double figure buyins por whatever), just play poker.