How does a recently repentant girl get rid of the feeling that God is displeased with her, that she commits disbelief (kufr) when she prays, and that she suffers from severe obsessive whispers and anxiety about dying while in a state of sin, along with her desire to enter Paradise and be saved from the torment of the grave and Hellfire?

As for intrusive thoughts (waswas), we advise you to strive against them and disregard them. They will not harm you as long as you dislike them, and you will be rewarded for your effort in trying to get rid of them. Regarding your repentance, Allah accepts it. It is not a condition for the validity of repentance from one sin that you must repent from all sins. So, if your repentance is sincere, Allah will forgive your sin. Allah Almighty says: "And He is the One who accepts repentance from His servants and pardons sins." Your communication with a non-mahram person over the internet is forbidden, and you must repent from it and cut off contact with them. Similarly, your failure to observe the prescribed hijab is forbidden. Among the things that will help you remain steadfast are contemplating the greatness of Allah, reflecting on death and the Day of Judgment, pondering Allah's Names and Attributes, striving in acts of worship and supplication, and seeking out righteous company.