There is can't which means 'this seems too complicated' or 'it doesn't work'. This usually relates to being nage.
My way to deal with this is remind myself of the happy surprise I feel when I suddenly realise that I just did something I had thought impossible - this comes after *training* again and again, + encouragement from my Sensei and asking about possible flaws in my technique - I like this feeling so I'll keep trying.
Sometimes I feel it's better to leave it for another day. After too many frustrations my brain may become locked.
Then, there is 'can't' which is related to fear.
I experienced it (again) yesterday. We practiced high falls, and I felt I'd be doing it wrong and that I'd be about to harm myself. I kept falling on my back, which increased my fear.
I was reluctant from trying again as I don't think a back injury is something I wish to risk. I feel I need to go back to lower rolls until I feel more confident with these. I won't do it until I'm sure I can do it without harming myself - it's different than doing a technique which 'doesn't work' - here it's me who might not work again !
A partly solution -my teacher encourages me to try but also to take things at my own pace - I favour this attitude. Still, it's frustrating to see others with no fear who fly around. I don't want to get rid of the fear but to be sure there's nothing to be afraid of (it ain't the same!).