heaven fashioned us of nothing,and we strive to bring ourselves to nothing- mentions the speaker and explain the meaning of the comment with reference to the context

Bosola brings them a letter from Ferdinand, which asks for Antonio to be sent to him, using double-talk so as to threaten his murder while pretending to offer amity. The Duchess sees through his “riddles” (3.5.41) easily, and so Antonio refuses to go. Bosola scorns his refusal and leaves.

The Duchess, fearful of an ambush, pleads for Antonio to take their oldest son to Milan. This is when the Duchess laments about being born to suffer under a divine hand,

"Must I, like to a slave-born Russian, Account it praise to suffer tyranny? And yet, O heaven, thy heavy hand is in't!"

Here is where your quote comes in. Antonio says that Heaven has fashioned us for suffering in this world and we oblige it (Heaven) with our own actions. Ourselves amounting to nothing seems to be God's plan for us all. That's my take!