When to let go

As the saying goes: Perseverance is victory.  While, a Buddhist says: Instead of fixating on something, let it go.  Don’t these two thoughts contradict each other?   Though a correct fixation might be directed towards success, a wrong fixation will lead to a dead end.  There are some people who struggle to walk forward, not because of the beautiful scenery ahead, but because of an unwillingness to abandon their previous efforts — just like someone who struggles in quicksand sinks deeper and faster into it.  As a result, one should differentiate between when to persevere and when to let go.