In order to be "stubborn" a dog has to absolutely, without question, know what you want, and then willfully not do it. At 18 weeks, she's not stubborn, she just hasn't learned yet. If you are "waiting 10 minutes for a sit", then your training is the issue, not her being stubborn.
Keep sessions short. Five minutes is a long time at that age. It's much better to do a minute here and a minute there throughout the day than to have a single 30 minute long session.
If you don't get a behavior within 10 seconds or so, help her by luring her into position. Mark and reward all attempts to do what you ask. Jackpot when she does something especially well.
Remember that dogs don't generalize well. Just because you have a "sit" in the kitchen doesn't mean she knows that "sit" means the same thing in the dining room, the living room , the bedroom, the back yard, the front yard.... well, you get the picture. You have to practice in lots of different places to help them realize that "sit" means" "plant your butt on the ground" no matter where you are.
The nervousness can be at least partly genetic, but it can also stem from not being properly handled during some critical developmental stages. When you are socializing her, don't force her to interact with things or people if she doesn't want to. Let her look at them from a distance she is comfortable with.
Also, since you have two pups that are the same age, look into littermate syndrome. It doesn't apply to just littermates, but to all dogs that are close in age.
If you don't have access to a good, positive based in-person trainer to work with, or even in conjunction with them, there are a couple of online classes that I think would help you. The first one is running live starting October 1, called Performance Fundamentals.
https://fenzidogsportsacademy.com/index.php/courses/49 The second is a self-study class called Raising a Performance Puppy.
https://fenzidogsportsacademy.com/index.php/courses/24 ALthough they both say "performance", they are good, basic classes that all dogs can benefit from.