There is an entire song about her creating the castle and why she did it. I'm not sure if you heard it or not but it goes. "LET IT GO! LET IT GO!" And makes you want to flush yourself down the toilet
you seemed to have had a lot of anger over that song and you LET IT GOOO very good
LET IT GOOOO
LET IIT GOOOOOOO
you couldn't hold your anger anymoooorrrre
She can't control the strength of her power; she can't keep it contained, but when she let's it go, she unleashes all of her power and the castle is made.
That's kind of how Disney movies work, really. I call it 'The Productivity Song'. Simba grew up and stopped giving any fucks, China's army was trained in the course of one song, Elsa builds a bad-ass castle, and so on and so forth.
Guys, she can't control her powers because she fears them and keeps then bottled up inside. It isn't a problem as a child because she had no reason to hear them, but after a she freezes Anna by accident she becomes afraid of them. When she is out away from the world she doesn't need to fear them, and had no issues with using them, but when Anna shows up her fear is renewed. Finally when she discovers that love could heal her sister she knew she didn't need to be afraid anymore. Hence she can use her powers perfectly after that.
LET IT GOOOO
LET IIT GOOOOOOO
you couldn't hold your anger anymoooorrrre