The flashbacks,part of your PTSD,are something you should be working on managing with your therapist.You can't stop them from happening but you can learn to manage them.On the other hand,if it's just thinking about the past rather than flashbacks(maybe that's what it is since you said it's stupid stuff?)then you can find ways to distract your thoughts.Or learn to just allow them to be there without getting caught up in them.

Getting stuck on conversations and obsessing and ruminating,finding ways to change your thoughts or letting it all go helps.That's so hard to do though. Two nights ago I was awake all night playing an argument over and over that I had with my husband.I just couldn't stop.What I should have done is meditated or imagined filling a bucket with all those thoughts and putting them away for another time.

It may seem like you have no control over your thoughts but with the help of your therapist or by reading threads about that kind of thing here at PC you can learn ways to help yourself with it.

My therapist suggested Seroquel to help my obsessive thoughts and rumination but I chose to learn ways to manage on my own.They don't always work but the more I practice the better I get at it.

It doesn't sound like either dissociation or maladaptive daydreaming to me.It sounds like obsessing and ruminating.I'm not a doctor though so be sure you talk to your therapist or pdoc about it.