The things people seem to keep forgetting about the lightsaber duels at the end are pretty numerous honestly. He had just killed his father, and was shot by Chewie of course which really smurfed him up. Anyone who thinks they didn't apparently wasn't paying attention. But there was more to his loss than that.

We also know at least two other things about him. Namely that his training isn't even finished and he's prone to losing his cool. A force user in general, even a Sith or Jedi aren't unbeatable. One who's not fully trained, injured, conflicted over his place in the force, conflicted over killing his father, and also enraged? Frankly he may be lucky he beat Finn. And if that aren't enough we're outright told his biggest fear is he won't live up to Vader and he's just been faced with a woman who is clearly able to more easily tap into the force, likely in part because she isn't uncertain about who she is and knows what she's fighting for. So clearly self confidence is an issue for him as well on top of everything else. And it makes sense that would cause issues since the only times Luke ever struggled with the force in the original trilogy are when his own mind and ego worked against him.

Long story short, there are a lot of reasons that he lost and almost none of them have to do with Rey being perfect. The only thing she really was when she turned the ride of the battle was calm and focused. Before that she was lashing out as wildly as he was.