Hi! I see you joined a year ago, so I'm guessing you've been struggling for awhile now? Your questions are good ones!
why talk about the details of trauma?
Well, it's not just talking about trauma that's beneficial. Yes, if you break the silence about the past, there can be a therapeutic effect, but the true healing comes from processing what happened to you. You can talk about your trauma until you're blue in the face, but if there is no processing, your healing will be limited.
why tell my therapist about the bad things?
Your therapist can help you best if s/he knows what you've experienced. If you're uncomfortable talking about it, then you could just say (or write) it in one sentence until you feel comfortable saying more.
what happens afterwards?
I'm a bit confused about what you mean by "afterwards"
what changes?
does my brain change?
Your brain won't change back to being like it was pre-trauma, but it can change for the better. This is a bit simplified, but what happens in our brains is that certain reactions, feelings, etc become entrenched in our minds. These are called neural pathways, and when we constantly react in a negative way, then these pathways are more likely to be used in the future. Kind of like a reinforcement of the negative stuff in our minds. With practice, we can actually change these neural pathways so that we don't automatically react in the same negative way. It takes work but yes, your brain can change.
does more exposure correlate with less anxiety?
In many cases, yes. But, it needs to be controlled as to not overstimulate and re traumatize you.
all I can think of is why?
I'm not sure why it works exactly, only that it does. I know that exposure therapy is what fixed my agoraphobia. I see so many people who shy away from what they fear and they only get worse. I had no choice but to face my fear and I credit my own version of exposure therapy for getting better. The same can be said for exposure therapy for PTSD.