Are you responsible for a girl's conversion to Christianity because you left her, which may have caused her to despair of God's mercy?

You should thank Allah for severing the relationship with the girl. Gratitude for blessings is shown by pursuing the path of faith and steadfastness in obedience, so that Allah may increase His bounty, as stated in His Almighty قول: "If you are grateful, I will surely increase you [in favor]". If the girl apostatizes from Islam, you bear no sin for that; for every soul is responsible for itself, as mentioned in His Almighty قول: "And no soul earns [anything] except for itself, and no bearer of burdens will bear the burden of another". You should cast away obsessions and dedicate your effort and time to what benefits you in this life and the hereafter. As for accessing her private account without her permission, it is prohibited espionage, due to His Almighty قول: "And do not spy".