My 15-week-old Dutch Shepherd puppy is hesitant and nervous around people, is this normal? How much should I push her with socialization? 

I feel like most people have a different definition of socializing than we do. Here's how we socialize our puppies: Socializing Puppies

I would highly recommend the video Raising a Puppy with Michael Ellis, it's going to cover all your questions about this as well as give you the right way to introduce her to things that make her worried. We focus all our early training and "socializing" on engagement with us so they learn to focus on us in spite of the environment. This builds confidence. This video is about 7 hours long and I recommend it to everyone with a pup, especially of the strong working breeds.

PS, the wrong kind of socializing can definitely be harmful to a young dog's confidence. I never want my puppies to feel nervous around people or situations so you have to be sensitive to her demeanor and body language. I also NEVER expect my sensitive or less confident puppies to directly interact with people and by trying to get her to do so you may accidentally make her apprehension about it more pronounced or a habit. If one person does the wrong thing and scares her at a critical developmental period you could be dealing with the repercussions of that long term.