The 'Who Says' hitmaker - who split from the 20-year-old singer earlier this year - is reportedly struggling to move on.
A source shared: "There are lots of guys interested in her. She's getting a ton of attention, but she's still so obsessed with Justin.
It's really hard for her to move on."
The 22-year-old singer - who had been dating the 'Boyfriend' hitmaker on/off for three years - is believed to have organised a gift for him this Christmas.
A source added: "Selena is all over the place when it comes to Justin.
"One minute she hates him and the next minute she's talking about getting him a Christmas present. Her friends are telling her not to buy him anything, though, because he has to be the one to make the move."
Despite coming up with Christmas gift ideas, Selena is worried the pair's split will ruin the festive period for her after she got "upset" about his new friendship with Hailey Baldwin.
A source revealed to HollywoodLife.com: "Selena is trying very hard to pretend that she isn't upset about Justin and Hailey, but the truth is it's killing her. She's having a major breakdown. It's putting such a dark cloud over the holidays.
"Selena's planning to be in Texas with her family for Christmas, but she's very worried that she won't be able to enjoy herself because she's so upset over Justin. She's having a lot of anxiety. Knowing that Justin is spending all his time with Hailey is eating her up."