“iʼm so tired/pissed off/depressed i can barely see straight.”
"see straight" literally mean see something in straight direction, but I didn't catch why it means "see clear" as figure of speech? thanks

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It's just an idiomatic speech pattern. "I'm so xxxxxx I can't/can barely see straight". Plug in the adjective of your choice. The condition renders the speaker unable to function properly.

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"Iʼm so tired/pissed off/depressed I can barely see straight.”= I am so tired that I can barely function normally.

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see straight = see the road clearly in front of you
Now extend that to thinking...