Is reading Surat Ya-Sin and supplicating for marriage to a specific person—after an unjustified separation from him—considered opposing God's will, and is there a correct way to read it?

Reading Surat Ya-Sin in the manner mentioned is an innovated Bid‘ah. It is permissible to make abundant supplications for Allah the Almighty to marry you to the person mentioned, as long as you believe him to be good and are reassured by his religious commitment and character. It is advisable to supplicate for Allah the Almighty to facilitate what is good and to grant a righteous spouse, without specifying a particular person.