Not sure if mentioned but the books basically comes right out and says that Kylo is getting weaker and weaker as the movie goes on. Because of his struggle with the light, and his emotions towards his father. That's why he tells Vader to show him the darkside.
Not to mention that Rey likely had some Jedi training as a kid. Of course she was very little, but still, it was there. The book also implies that Kylo cares about Rey / has compassion for her. So he wasn't trying to kill her.
And like others have said, Kylo was hurt, he wasn't 100% in the battle at all. Mentally, or physically.
And I don't see the reason for the complains regarding his fight vs Finn. Finn was completely outmatched vs Kylo. Ren was just toying with him. Like Maul did vs Obi-Wan... Like Vader did with Luke (using a one hand stance and then switching to a two hand stance once he realized he couldn't play with Luke) and like Sidious did to Luke, torturing him with lightning for minutes before he decided to kill him. Dark side users are known to play with their opponents only for it to bite them in the butt.. So Kylo doing it with Finn is no surprise, he finished him when he was ready.