It's almost as if he took a bowcaster bolt to the side prior to the start of the fight. A weapon, that I might add, was hyped up constantly throughout the film.
And again, if were taking ROTS Kenobi, he beats down Kylo pretty solidly. Kylo Ren was a big fish in a small pond, a powerful force user when there were basically no other force users, people like Maul, Sidious, Vader, Kenobi, Yoda etc, were all top level force users when the galaxy was full of them.
Even if everything you say is true, it's still a very poor showing for him. He lost to somebody who wasn't even a Padawan yet, and even if we buy into the (she has a strong connection to the force) than we'd have to take that to its logical conclusion... which still isn't good for Kylo. In other words, his force potential is always below here...
This whole confusion is down to the Inconcsistency in Force powers this movie has shown us, and forcing in a Saber battle into the film that really should never have happened.