It is okay to not know everything in development and it is okay to ask for help
Right on. And this remains true for your entire career! The smartest people I know are always the first ones who admit when they don't know something - that's how they get better. You can't learn from your mistakes if you never allow yourself to admit to them!
About the imposter syndrome... I know it sucks to deal with the (sometime crippling) doubt -- my wife & I still talk about this, 20-odd years into our careers. But you never want that feeling to go away completely. Part of what causes that feeling is when you're outside of your comfort zone, and that is the only way you will continue to grow. It can be tough to push through it, but being brave isn't about not being scared, it's about doing what you need to do in spite of the fear. I'd like to say it gets easier over time, but I think it's more like a habit you have to develop over time.