How to increase sex in my relationship

I am in desperate need of advice to increase sex in my relationship. Me and my girlfriend are both 19. When we first got together, we had a lot of sex, and it was amazing. We are now four months in and the sex has dropped to once every two weeks. Initially when she would deny me I would become visibly upset and she would see that. I then figured out it’s best not to show it. Now I am not sure what to do. I am thinking it’s maybe because she no longer feels attracted to me. She has stated she feels like she wear the pants in the relationship. I have also cried in front of her many times and expressed my emotional side a lot. I have since stopped that. Do you think the sex has stopped due to me not showing enough masculinity and assertiveness? Either way, I would love to here your input and find something I can do.