Diminished Reality (DR) modifies or removes elements of the perceived environment. We explored DR for attention support through diary studies, interviews, and speculative co-design with 15 university students with ADHD. Participants described a persistent gap between intention and attention, and the use of externalization strategies that encounter fundamental limits in physical reality. In co-design, rather than reasoning from visual operations, participants articulated which properties of stimuli they needed to perceive versus filter, revealing cases where visual diminishment does not reliably map to attentional diminishment. We contribute rich descriptive data on the attentional challenges and DR preferences of students with ADHD, and propose Attentional Diminishment, a sensitizing concept that reframes DR for attention support from visual outcomes to attentional effects.