Is my dad depressed or is it common for people with dementia? Every day he says he can't take it anymore, he can't stand it, he can't stop.

Tookie, if you're able to see a specialist like those the other commenters suggested, I would definitely do that. Primaries, even well-loved and trusted ones, are jacks-of-all-trades but masters of none. A specialist will have a deeper knowledge of dementia behaviors, causes, how to treat for it and newest and best info, as well as familiarity with drug interaction (which is a common issue for seniors).

When my 90+ yo aunt was in the late beginnings of her dementia she'd weep and lament loudly, "Jesus! Help me! Dear God I can't take it anymore! Help me God!" over and over. It was so distressing. Her sister finally got her on some meds for her mood and it has helped her a lot (not to mention it helped the sister)!

If you aren't confident in your regular doctor then perhaps a gerontologist, geriatric psychiatrist, or a neurologist specializing in his form of dementia.

It would be worth a try certainly.

I hope you find him something that helps his torment.