Presley Gerber's last girlfriend is remembering their final months together ... sharing selfies and love notes in a touching tribute after his death.

Charlie Manley posted a carousel Monday night with several selfies of the couple, a handwritten note from Presley, a bouquet of flowers, and a video of waves crashing.

Charlie called Presley "the most incredible and inspirational human" she'd ever known and said she feels "endlessly lucky" to have loved him -- and been loved by him -- during his final months.

She says ... "What we shared is something I will carry with me for the rest of my life. A true angel. Your beautiful heart and soul will live on with me, forever and always."

As we reported ... Presley died Sept. 20 after checking into a Santa Monica sober living facility. His official cause of death remains under investigation, but we're told authorities believe he died of an overdose.

We're also told Presley's family has been considering a private memorial at the Gerber family's Malibu home, where loved ones could gather privately to remember him.