Usage patterns
メンタルが強いto be mentally strong/resilient
He's mentally tough, so he bounces back quickly even after failing.
メンタルが弱いto be mentally weak/fragile
She's mentally fragile and starts crying as soon as she's scolded a little.
メンタルをやられるto be mentally worn down/broken (by stress, criticism, etc.)
The daily pressure ended up wearing me down mentally.
メンタルを保つto keep/maintain one's mental state (stable)
It's important to stay calm and keep your mental state steady during the match.
メンタルな〈事〉a mental/psychological (issue, problem, etc.)
I think that's a mental (psychological) issue.