I'd say that you need obedience training from a professional trainer that is referred by someone you trust.
Some dogs are really easy to train, but this does not seem to be the case here.
By the sound of this, and with respect, it seems like this training problem is not something you can handle yourself. Your dog is not respecting you as the leader. Your dog will grow bigger and stronger. Both the dog and you will benefit from group classes with other dogs and a trainer that knows how to deal with dogs like this.
Normally these classes aren't a fortune and imo, it is THE single best investment you can make. Trying to deal with these problems later in life (your life and the dog's life) will be far more difficult and lead to way more frustration.