Ex-NBA player Jason Collins -- the league's first openly gay player -- just opened up on his serious health battle ... revealing he was diagnosed with Stage 4 glioblastoma, "one of the deadliest forms" of brain cancer.
Collins, who announced he had a brain tumor earlier this year, detailed his journey in an interview with ESPN ... saying he first noticed symptoms back in August.
The biggest issues he was dealing with included short-term memory loss and depleted comprehension .