system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars ROTJ] Did Anakin have trouble fighting Luke because he couldn't feel enough hatred towards Luke to fuel his Dark Side Force use?
gpt
Pretty much, yes.  

Ever since Vader was informed that the pilot who destroyed the Death Star was named Skywalker, he had become obsessed with finding Luke. For much of the war post-Yavin, Vader threw his entire being into finding what he thought was his last connection to Padme. Luke was living proof that he wasn't alone, and that many of Palpatine's words were outright lies. Anakin had been distancing himself from his Sith master for quite some time before Luke, but his son was the big push he needed to start making plans to take Sidious down. Vader just didn't realize that pursuing Luke was also his path toward redemption. 

By the time of the fight on Endor, Vader was struggling mightily with his dark side persona and the one he called "the Jedi", a self-imposed lockbox of all his memories and feelings as Anakin. After his failure to capture Luke on Bespin, since he had promised to capture Luke and make him "a powerful ally" of both Sith lords, Sidious punished and tortured Vader mercilessly. It's why he was so cowed and seemingly hopeless when Vader brought Luke to the second Death Star. 

Vader half-assed the fight in the Emperor's throne room partly because he was following his master's instructions to do a grand pantomime in the hope of turning Luke to the dark side. But the other part was exactly as OP asked, his waning grip on the dark side and the resurgence of Anakin Skywalker. When Luke started drifting into the dark side and beat Vader into submission, he genuinely won that fight, because Vader was stuck in such a deep crossroads of his old and new personas struggling mightily in his head. It wasn't until Anakin saw his son being tortured and killed, his last family being taken from him, that Anakin won the fight once and for all.