During the fight between Kylo and Rey, Kylo was beating ass because Rey was panicky and unfamiliar with the weaponry. But once she reached for the force, I took that as her calming her mind down and getting back to the basics of fighting she knows. That, along with everything else previously mentioned (being shot/personally distraught/et cetera)went into her defeating Kylo there.
When I was wrestling and practicing jiu jitsu/muay thai, one of the things I always had my coaches/trainers tell me was to get really good and comfortable with the basic stuff. During wrestling matches and fights, my corner man was there not only as an extra set of eyes, but to say things to help keep me calm. The Marines I teach martial arts to I always tell that the most important thing is being calm during a fight and practicing 'brilliance in the basics'. Being calm and knowing the basics techniques really well is the best way to win a fight.
I know some people have been upset about Kylo's appearance and the actor who portrayed him not looking like Han or Leia. But guys, remember, he has too much Vader in him. I'm pretty sure he is designed to look more like Anakin than his parents, and I think that works for him. He is not only as emotionally unstable as Anakin was, but he has a similar look as well, and not just the one with the mask that he copies from Vader.