I don't think it is a mental issue. She got scared once and has now developed an uncomfortable and unhealthy habit. She may not be using the right laxative for her problem. You might give her stewed prunes on a regular basis. This is an old-faashioned remedy that would cause a mild, but not painful, diarrhea. It might break her habit without harming her.
Also, check out the poop. If it is still hard then the miralax is not doing the job and she needs something else. She remembers the pain and I am thinking that it still must be painful. Also a tiny amount of blood suggests hemorrhoids which could explain why it still might be painful. You might want to have a doc check her out.
My son was theexact same way. Don't make an issue of ut to her or in front of her because its already an issue fir her in some way. If she sees you fretting she will fret. My first step was ignore it to her face. Don't encourage or discourage. See if she will just eventually go when left to it. If this doesn't work get a treat jar and put it up somewhere nearby. Whenever she goes she gets a sticker/candy.