Luz thought that Trench was always putting a wrench in things, but she couldn't help thinking it had to be her fault. She kept intending to try and help Usagi, but instead wound-up saying things that sounded more like they were about HERSELF, and obviously Usagi noticed this. To Luz, Usagi cared about people more than herself, and failing to help her meant she wasn't doing what she was supposed to.

Even if she seemed to help others, failing to help one person meant to Luz she was doing something that wasn't working.

"Another way? How? Will we have to take it apart?"