My first novel is trash, but that's okay.

I just finished writing my first book. I should be happy, but all I see are the flaws. My dialogue was garbage, my sentence structure was wooden and bland, and I feel like nearly every sentence started with "She did, He felt, etc." I can see where I need to improve, but now how do I fix it?

I am not the brightest crayon in the box, so just someone saying, "Go listen to people, and watch how they talk," isn't going to help me much. It may be autism, but I have never been good at observing people. I have been reading and rereading books trying to pick out what hooked me on them in the first place, and how they flow so well, but I think I am missing something.