I'd write: He tries hard, but even with the best will in the world, I doubt that he'll succeed. I doubt that he succeeds. ) "On one's mind" means that a peson thinks about something a lot.
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RuslanaI'd write:Thanks!
He tries hard, but even with the best will in the world, I doubt that he'll succeed. (Or it can also be: ....I doubt that he succeeds.)
"On one's mind" means that a peson thinks about something a lot. So your interpretation is right.