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Demi Lovato 's ex-fiancé Max Ehrich says he's going to rehab after his domestic violence arrest ... and it sounds like he needs help, based on what he's telling us about the leadup to his arrest.

Max tells TMZ ... he was high and in psychosis when he had an altercation with his mom that resulted in him being hauled off to a Florida jail on a battery charge. He says his mom was trying to get him to stop huffing nitrous oxide. Play video content

The "Young and the Restless" actor says he went to Florida to stay with his parents ... after allegedly being kidnapped in Los Angeles last week by 3 gang members. He says his alleged kidnappers introduced him to nitrous, and he was high on nitrous for a week leading up to his arrest.

Max says he is under a 72-hour psychiatric hold i

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