In case you didn’t get it
'Something concrete' is actually a metaphor, 'whenever I wanted to do something concrete' means 'to be who I wanted, to follow my own path'. Kaneki could experience it with Hide because Hide accepted him, regardless of Kaneki’s desires or unattractive traits. So after Kaneki put distance between them, whenever there were times he felt like 'doing something concrete' again, he'd want to see Hide again - as if to share this new feeling of being confident and comfortable with himself, to reunite this feeling with that same comfort, joy, 'home' he had with Hide. But he was desperate, and afraid, and he couldn't endure it ('But whenever I did, I'd want to see him so badly I couldn't help myself - better translation) - endure the thought of being rejected and losing this feeling altogether.