Eric Dane Tears Up Over ALS Battle, Says He Has Only One Functioning Arm

Eric Dane is facing the brutal truth of his ALS diagnosis -- and it's hitting hard, 'cause he got choked up revealing how he's now down to just one functioning arm ... and even that might not last much longer.

The "Euphoria" star dished all on "Good Morning America" ... explaining while his left arm is still working, he can feel it getting weak, and knows that one will probably be out of commission in a few months too.

It’s not just his arms -- the 52-year-old says his legs are next ... recalling a terrifying snorkeling trip with his daughter where he ran out of strength, and she had to pull him to safety.

Eric says he's angry about the diagnosis -- especially after losing his own dad to suicide at age 7, he now fears his his two daughters might lose him way too soon.

It’s been a lot for Eric to process since revealing his ALS diagnosis back in April -- especially after the exhausting road just to get accurate answers. He bounced from one specialist to another, test after test ... until a neurologist finally dropped the bomb: ALS (amyotrophic lateral sclerosis) -- three letters he says haunt him from the moment he wakes up every day.

Despite it all, Eric held it together -- until he brought up his wife, Rebecca Gayheart, being his rock, his biggest support ... and that’s when the waterworks hit.