Justin Bieber is rumored to be beside himself with envy over Selena Gomez's budding relationship with Samuel Krost.

Hollywood Life reports the "Come Get It Singer" still weighs heavily on Bieber's mind, and he simply can't bring himself to think of her with someone else.

"[Justin] is trying to avoid all the Samuel Krost stuff because it makes him jealous," a source said. "He hates that Selena has finally figured him out. He took her for granted and it is now coming back to haunt him."

As previously reported, Bieber and Gomez were last publicly seen together in late November, when he serenaded his ex at a Beverly Hills club with a rendition of "My Girl." Since then, he's openly insisted he's convinced his ex still has feelings for him and always will, despite their many makeups and breakups.

For a while, the two were reported to still be keeping in regular contact, but now sources indicate Gomez has started to drift away more than ever, at times even ignoring Bieber's calls and texts altogether.

In recent weeks, Hollywood Life adds Gomez and Krost have been seen all over New York together, each time seemingly a bit more at ease with one another.

Meanwhile, Bieber has recently been romantically linked to both the much older Kourtney Kardashian and teen model Hailey Baldwin. As Latin Post also reported he and Baldwin spent a romantic New Year's Eve in the Caribbean.

Still, Bieber's every thought seems to be about Gomez.

"It really is screwing with his head because he is not in the driver's seat with her," a source added. "It really is something he isn't familiar with and he is trying to figure out if he should let her go for good or not."

Before Krost, Gomez was rumored to be dating One Direction star Niall Horan.