“16 and Pregnant” star Whitney Purvis left a lengthy and emotional message on the online obituary page of a man who fatally overdosed on drugs she allegedly supplied … TMZ has learned.

The heartfelt comment, posted on March 3, was written weeks after the February death of John Mark Harris — who authorities say was given deadly drugs by Purvis prior to his death.

Purvis wrote … “John mark, I hate that I found out what happened today and missed your funeral. I wish I could’ve found out sooner and attended your celebration of life. thank you for being such a great friend to me and always making me feel beautiful and cared for.”

She went on to describe their close friendship in vivid detail — recalling their shared love of cooking, TV binge sessions, smoking Camel Crush cigarettes together, and long conversations about life. “You came to my rescue on many occasions and saved me over the years,” she added. “I could tell you anything and you always knew what to say.”

WP said she’ll miss his amazing cooking and miss snuggling on the couch watching TV with John. “Most of all, I’ll miss you, a truly good-hearted & one-of-a-kind man, John Mark.”

Purvis ended with … “Rest in peace and my condolences to your family and friends. I pray they are able to find some kind of comfort during this tragic time. I love you forever, John Mark. Love, Whitney”