Hang in there, OP. It's admirable that you want to help. I can relate.
Your title made me do a double take: I also have a 37 year old brother who struggles in life and I know it's a strain on the family. My brother does not seem to be as "bad" as yours, but lives so far under what others see as his "potential" that it bums everyone out.
After graduating high school top 30 in his class, with lots of AP classes and very high SAT, my brother failed out of college, and has spent the last 20 years doing poverty jobs like fast food work or carpenter's helper (not a carpenter himself mind you) or being unemployed. He has not worked since pre-COVID. He did some recreational drugs during his college failure days, but has not been an addict as far as I know. He has no criminal history.
He has been obese his entire adulthood, and I know he has bad sleep apnea, but he refuses to get it diagnosed or treated.
He is a decent guy, just seems depressive, and totally lacking in drive or ambition to improve himself. He doesn't work, he doesn't exercise, what does he DO all day every day? I don't know.
I offered multiple times to pay for any and all costs to address his sleep apnea or find a career and he has declined.
His situation drives my mom nuts and I have finally told her (and come to the painful realization myself) what others have posted on this thread previously: there is NOTHING we can do unless they want to improve themselves. Despite her wishes for him, he is a shclub UNTIL PROVEN OTHERWISE.
Jackson Browne wrote a great lyric (esp for a 16 yo) in his song "These Days"-- "please don't confront me with my failures, I had not forgotten them."
I have decided to stop offering help because it is too painful for me to have him refuse, and like the song above says, he is aware of what he has become. My plan going forward is to simply show him love, and accept him for what he is, until and unless he ever indicates otherwise.