Mod Sun Devastated Over Split from Avril Lavigne, Blindsided by Tyga Romance

Avril Lavigne's looking carefree with her new BF, but her ex-fiancé, Mod Sun, is having a hard time watching her move on so quickly and so publicly.

Sources close to Mod Sun tell TMZ … he's been down in the dumps since his split from Avril, which, as we first reported, was far from a mutual decision. As if being blindsided by the abrupt end to their engagement wasn't bad enough -- we're told he feels absolutely devastated by her budding romance with Tyga.

Avril and Tyga have been together all over Paris for Fashion Week, and they're pouring on the PDA pretty heavily... we just shared shots of them locking lips Monday night at one event.

Those pics are especially triggering for Mod.

Our sources pointed out Avril and her new guy are making out in the same city where Mod got down on bended knee to propose almost a year ago -- so, he views Avril parading around Paris as a huge slap in the face ... or even vengeful toward him.

We're told he feels he did nothing wrong, which is what makes Avril's new relationship a tough pill to swallow.

TMZ broke the story ... Avril recently called off their engagement, just a few days after Mod started touring in support of his "God Save The Teen" album.

According to sources close to Avril, they'd been "off and on" for months, and it just wasn't working -- but Mod sources claim they were together without interruption and living together.

You'll recall, MS recently opened up about the breakup on IG ... saying his fans and tour life were helping him get through it all.