Where I am coming from is the idea that anyone can do anything, but some just require more training. Some things come naturally to people without them having to be trained. Sometimes training in one area actually prepares you for something completely different. Early in the days of computers, they were looking for people with musical talent because they found a connection between understanding music and understanding programming.
So let’s look at how this impacts our Jedi characters.
Classic Republic Jedi are trained from a very young age. They are indoctrinated into the Jedi philosophy and trained in the Jedi arts. They are basically raised in a monastic order - never supposed to develop attachments and remain aloof.
Anakin is found at the age of 9 and is already strong in the force. He has tapped into the force to become one of the best Pod Racers (at 9) in the galaxy. That translated to flying a fighter. In the decade he is trained by Obi-wan, he is school in the Jedi way and the Jedi arts. We have no inkling how easy or hard it was for him to master any of the Jedi arts, but it is made very plain that he chafes at the Jedi way. It is clear that the Jedi training that they rely on to keep Jedi from being tempted by the dark side did not work. Obi-wan feels he failed Anakin, but we see that Anakin had issues and was being mentored by Palpatine which contributed to the Jedi teachings not being effective. But even though they were not effective, he was still one of the most powerful Jedi of the period. Obi-wan felt that if he applied himself more to his training he would rival Master Yoda. Anakin appears to have been conceived by the Force as part of a prophecy of balancing the Force.
Luke starts his training at 19. He has also tapped into the force for flying, becoming the best bush pilot on the outer rim, according to Biggs. He grew up in a very practical home, but he mastered deflecting blaster bolts from a remote in one lesson. Yet he had so many doubts that he could not lift an X-wing. Yoda said his lack of belief is why he failed. We never see Yoda train him with a light saber at all. We don’t see Yoda teach him any skills except communing with the Force and lots of physical activity. Yet in Return of the Jedi he has mastered them all. The amount of training he had depends on the hyperspace travel time from Tatooine to Aleraan and the non-hyperspace (or by backup hyperdrive) travel time from Hoth to Bespin. That is day or months, not years. So Luke did not receive long and extensive training. According to the official timeline, it was 4 years between ANH and ROTJ. But however long or short, Luke was able to resist the dark side far better than his father who had a decade of far more intense training. Of course he didn’t have a Sith Lord whispering in his ear for that same amount of time. Luke was the biological son of one of the most powerful Force users of the old Republic. He seemed to have inherited some of that. He had no known training in levitating objects before he grabbed his lightsaber to cut himself down in the Wampa cave. He hadn’t even seen anyone do that. So he exhibited a self taught skill.
Rey is of unknown age (Daisy was 22 when she filmed TFA). She has been living on Jakku since she was a kid. She has been working for an unknown time. She is a scavenger so she has crawled all over the wrecked ships of Jakku (we see a couple of Star Destroyers, a Super Star Destoyer, an AT-AT, and an unknown number of fighters) retrieving useful items. She is obviously a skilled climber. We don’t know who her parents are. They appear not to have been anyone of significance. She helps Finn escape and comes to Kylo’s attention. Through his contact with her she is exposed to the Force and she is able to pick up the skills that she sees Kylo use. Not without some difficulty. Luke helps her refine her ability to tap into the Force and she is able to life a bunch of rocks to free the survivors on Crait.
There are others, such as Chirrut Imwe, Ezra Bridger, but they aren’t in the Saga movies and seem to have a more difficult time picking things up than Luke. Plus there are ones we don’t know the training of like Ben Solo/Kylo Ren.
What is clear that amount of time training does not equal ability in the Force. If so, Obi-Wan would have been far more powerful and from the dialog it is clear that he knows his padawan is more powerful than he is. Luke, with so little training in comparison, is able to defeat his father in lightsaber combat. He is immune to the temptations of the Emperor (to join him - he is not immune to the threat to his friends). So we get to Rey. Rey doesn’t spontaneously learn skills she has not seen (like Luke levitating his saber). She is very powerful, but she is observing (not just with her 5 senses - with the force as well) another very powerful Force user and copying. We see her learning these skills in TFA and she is a natural. How does she master them? The force awakened in her. Why it chose her we don’t know. I think it was that it was dormant because no one around her had any trace of ability and it came to life in the proximity of Kylo Ren. She lacks the self-doubt and whininess that Luke exhibited in the OT. What she needs is a teacher to show her what she can do and a guide to help her use these newfound powers the right way.
Anakin’s journey was never about if he could learn these powers, it was about him trying to fit in to the Jedi while being lead astray by Palpatine. His journey is one of temptation and fall. Luke was a white knight when it comes to that. His one temptation was when Vader threatened Leia. Other than that he was as solidly in the light and his journey was to find the faith that he could do what he needed to do. Rey’s journey, although incomplete at the moment, has nothing to do with struggling to learn, but to find the path. She has chosen the light, but not the one sided light that young Luke had taken. She has the skills, but she has no future. What is she doing? What path is she on? Where will she take these newfound powers. Her journey is to find her destination, her purpose. Her force ability isn’t the issue in her journey, just like it wasn’t for Anakin. It was for Luke, but he had a clear purpose.
So worrying about how easy it is for Rey to pick up these skills and saying she isn’t a rounded character ignores her struggles and how they compare to Anakin and Luke. All three have a different facet of the journey to being a Jedi that they are struggling with. But all three have very similar journeys with obstacles to overcome.