The singer/actress has been responding to folks online about her relationship with the music producer ... and declaring, "He is my absolute everything in my heart."

She didn't stop there ... Selena took time to reply to people's less-than-positive comments about her dating choice -- saying he's "the best thing that's ever happened to me," and adds he's "better than anyone I've ever been with. Facts."

Selena clapped back at another hater with, "I will never allow your words to guide my life ... If you can't accept me at my happiest then don't be in [my] life at all."

While most of the back and forth has since been deleted, people were claiming Benny was a downgrade in terms of BF material for Selena. You'll recall, she's been linked to guys like Justin Bieber, Taylor Lautner and Nick Jonas.

In terms of her previous relationships, Selena simply told one commenter, "I'm not going to be with a f***boy ever again."

No, she was not specific as to whom she was referring with that remark.