Here are some suggestions: Family members should laughingly point out their own mistakes and good-naturedly go on from there. For example: "I can't believe I turned left at that street instead of right. Now how can I fix this!?" Do not give her a lot of sympathy when she cries over her writing. That just reinforces it. Be matter-of-fact and less consoling. I know this is hard to do, but she's letting her emotions get out of control at this point so you have to keep tabs on yours.
If she is truly having some problems forming her letters or writes too slowly to keep up with her speedy brain, teach her the "speedwriting" game. Have her pick a few sentences from her favorite book, then copy them down as fast as she can while you time her. Neatness doesn't count. The idea is to beat your own time. Use the same passage once a day for three to four days and then get another one. If she improves her time, then she gets a little reward (like more computer time). Explain that this silly but fun game will make her writing hand a little stronger by exercising it. It can often help to loosen up the pencil grip while strengthening the hand overall. I would also de-emphasize perfection in general with her. Mention this to her teacher as well so that she can do the same.