Katy Perry Feels Betrayed By Russell Brand !

Whenever Katy Perry gets around to writing new stuff for her third album, she gets carried away by plenty of dark, painful memories.

As many sources confirm , her estranged husband Russell Brand didn't simply blindside her with a Dec. 30 divorce filing; despite public denials from a rep, the British comic, 36, plans to cover all the nitty-gritty, sometimes ugly details of their failed 14-month marriage in an upcoming memoir.

"Katy is insulted and feels betrayed that Russell has treated her so poorly," a pal to the "Firework" singer, 27, claims. "She will never forgive him. Ever."

Although Perry is in panic mode regarding her ex's memoir plans, she's back to work on her ongoing concert tour, commercial shoots and more -- and she's taking pains to "limiting…interactions" with Brand, another source states.

"He's being so atrocious that she feels like she never knew him."