Jon Jones is DONE as a competitive fighter. Finished ... this according to Skip Bayless, who says it absolutely breaks his heart.

The FS1 star says he's gotten to know Jones through appearances on "Undisputed" -- and tells TMZ Sports news of JJ's positive steroids test crushed him.

"How can that guy be either so stupid, or so flawed, and ultimately so shockingly insecure, that you have to resort to steroids or whatever the PED was -- or any kind of drug?"

"He's just like a flawed human, but he's got so much potential. He's such a likable, nice guy."

In his parting message to Jones, Skip offered some tough love.

"Jon: I have tried for years to love you, and you just wouldn't let me love you. And in the end, I can't have any sympathy for you because you could've been the greatest."

Skip also looked back at JJ's appearance on "Undisputed" before his fight at UFC 214 ... and told us the way Jones acted seems pretty fishy now.

HE WAS PARANOID
TMZSports.com