Is the repeated heartfelt resentment of "I hate life" after watching pornography and masturbating considered an act of disbelief (kufr)? Does it necessitate performing ghusl (ritual bathing) to enter Islam? What is the ruling on prayers performed thereafter?

All that you are suffering from is intrusive thoughts (waswas), so do not pay attention to them. You have not disbelieved because of the intrusive thoughts and ideas that entered your heart; rather, you are still Muslim. Delaying prayer beyond its prescribed time is not considered disbelief by most scholars. You must repent to Allah for the sins you have committed, such as masturbation and negligence in prayer. You must also strive against these intrusive thoughts and cast them away, not indulging in them, for indulging in them leads to great evil.