Miley Cyrus' breakup with Liam Hemsworth has reportedly "messed with her mind".

The 21-year-old singer met the Australian actor while they were shooting The Last Song in 2009. They had an on/off relationship before he proposed in June 2012, with the engagement ending last year.

Miley is currently on her Bangerz world tour where she's been cavorting on stage in barely-there outfits and with a giant blow-up penis.

"Miley is not over him. The whole downward spiral started when things started to go south between them. All the breaking up then getting back together really messed with her mind," an insider told British magazine Heat. "This time last year, they were engaged and she was planning to be a devoted wife, stepping away from her career to follow him onto film sets. If things had worked out between them, you wouldn't be seeing this behaviour from her."

The claims come after Miley launched a ferocious attack on an ex while performing in London, UK. She later took to Twitter to insist her comments weren't directed at Liam, although they showed she still harbours a lot of anger about her love life.

"Every time you get in your car, you're going to hear my f**king song on the f**king radio, you piece of sh*t," she fumed before her Wrecking Ball performance.

"So I'm gonna tell those motherf****rs that broke my heart, particularly one, to suck my fat d**k and to enjoy hearing this song for the rest of your life."

The star has made no secret of her use of marijuana in the past and it's been claimed she is now experiencing mood swings.

"She's wired during the day but sleepless at night. She is not in a great place at the moment," the source said.