This idea of balance has eaten up a lot of time in Clone Wars and Rebels. Since Filoni was working closely with Lucas, I believe what we have been seeing represent development to the ST and a final balance to the force… back to where it all started.

So Rey is who she needs to be, has had plenty of setbacks and definitely not an easy time, regardless of how quick she has picked up the force skills, so she is not a Mary Sue. She is the main character of the ST. She drew a lightsaber from a box after helping Finn and BB-8 and her life has changed.