Young keepers are often chronic fuck-ups. Courtois was always kind of sporadic in his first years at Atleti. De Gea was also a confidence-case for United early on, especially with the lack of full trust and rotation with Lindegaard. When they were given the leap of faith from their coaches, they ended up being great.
I maintain that we're not going to see ter Stegen truly hit his stride until he's given La Liga. Much more consistent playing time with matches that boost confidence and build momentum, coupled with some challenging tests. In the CL he's facing the most pressure psychologically and has to play elite teams trying to show their best side.
He's got plenty of potential but he's still raw and it shows. Having Bravo take CL this year and giving MAtS la liga would be the best option.