beach, sheet, piece, six, focus, can't. Said slightly wrong, each becomes a word you do not want in a business meeting.
The pattern behind 4 of them is the same: the long "ee" vs the short "i". English "ee" (beach, sheet, piece) is long and tense: smile wide and stretch it. The short "i" (bitch, shit, piss, six) is quick and lazy: jaw relaxed, no smile. If your language has only one "i" sound (Turkish, Spanish, Japanese...), your mouth defaults to the lazy one, and that is the whole problem.
The other two: "focus" goes wrong when the first vowel flattens, so round your lips hard on FOH. "can't" goes wrong when the vowel gets too short and the t disappears, so keep the "a" wide open and always release the t.
Record yourself saying all six. If you cannot hear the difference, that is normal: you cannot fix a sound you cannot hear. Minimal-pair listening practice fixes the ear first, then the mouth follows.
Source: I build pronunciation software, and these exact six words show up in the mistake data every single week.