Ughhh me too! To me, the middle is the hardest part, because i'm never sure if things are moving to fast or too slow, or if I have too many subplots, or not enough. It's hard to tell when you're writing/reading your own writing. I think just get as many words as you can on paper, making sure you reach the ending you want, and then ask someone to read it through and tell you whether or not the plot is clear. If you don't want anyone to read it, write the whole thing and then wait a month. Come back to it with fresh eyes and read it. Then maybe you'll be able to figure out the plot. Another method that's helped me is the diagram (forgot what its called):
First, the set up. This is where you introduce characters and provide background –so world building and relationships should be the focus of this section.
The inciting incident is next. Basically, it's your protagonist's "call to action" or what kicks the plot into high gear. Like in the Chronicles of Narnia, Lucy finding the wardrobe would be the inciting incident because it starts their adventures in Narnia. In Harry Potter, it's when Hagrid tells Harry he's a wizard. Whatever starts the novel.
Rising action. The repercussions of the inciting incident, what your character is going to do about it, the stumbling blocks they face, etc. The meat of the plot. It should have increasing tension, increasing suspense, and a few side plots that tie back into the main plotline.
Climax. Biggest, most important part. Basically another inciting incident, but worse and with more repercussions. In the inciting incident, your character was innocent and hadn't experienced anything –now, at the climax, they've learned and are hardened by everything that's happened. The climax is the big battle, the highest point of the novel, the explosion of action and tension.
Falling action. The aftermath. How was your character changed? Where are they going from here? Did they win? Lose? A winding down from the climax to reach a satisfying conclusion.
I hope this helped some. It helped me develop my plot. Good luck.