When I open my main menu in my game, I cannot move the mouse anywhere at all, It just stays in the center of the screen
Try the Enable input node:
https://docs.unrealengine.com/latest/images/Gameplay/HowTo/ActorInput/Blueprints/input_1.jpg
More information about this node can be found here:
https://docs.unrealengine.com/latest/INT/Gameplay/HowTo/ActorInput/Blueprints/