The Force seems to be ready to use and good to go. The trick seems to be finding out that your are capable of using it and believing in it.
I think that's the jist of what Yoda was trying to teach Luke. Luke was completely resistant even though the dude blew up a Death Star using it.
These two posts can't be quoted enough.Oh man, I didn't even think about that. That may be the best way to look at how Rey was able to accept it so quickly.
Yoda's "That is why you fail" line pretty much lines up here as well. She goes through her vision and then has the Force directly affect her through Kylo's probing. She is quicker to fully accept the Force than Luke was in A New Hope because she's directly touched by it. Luke sees Obi-Wan use it on Stormtroopers, but he doesn't feel it like Rey does.
Also, as she read Kylo Ren's feelings and he admits to Snoke that he feels her strength, she's more inclined to believe that she is capable of great things.
edit: "yes, please" at the below