Justin Bieber and Selena Gomez. Picture: Bang Showbiz

American star Selena Gomez's relationship with Justin Bieber is in a "constant state of flux", according to a source.

The 25-year-old singer-songwriter and Justin, 24, rekindled their on/off romance in 2017, but the constant ups and downs in their relationship is making it hard for them to take things to the next level. 

An insider explained: "They are in a constant state of flux when it comes to their relationship. 

"When they are together it starts out great and when they aren't they always miss each other but they also want their own lives. Its a bit of a cake-and-eat-it too situation." 

It's been claimed that the celebrity duo remain together, in part, because they feel like no one else can really relate to the stresses they both deal with on a daily basis. 

The source told HollywoodLife: "They can't really put it all together. They will continue to break-up and make-up because they are together because they don't feel anyone else understands them. So it is a constant cycle." 

Selena's love life is reportedly complicated by her mother Mandy Teefey's opinion of Justin, as she worries about her daughter being hurt by the 'Sorry' hitmaker. 

Despite this, Mandy has observed some changes in Justin since the first time he was dating the brunette beauty. 

The insider added: "Selena's mom monitors it all because of what Justin did in their first go around. 

"So that will always be there but she has also seen some change in him and hopes that will continue. Selena's mom is letting go of the leash a little bit but still has an ample grip if Justin goes back to his old ways. 

"She is absolutely still in the mix because she wants the best for her daughter."