(Cover) - EN Showbiz - Rihanna is apparently worried that she will never find "true happiness".

The singer has been posting a series of messages on her Twitter account as she takes a break from work. In her latest series of cryptic posts, she appears to show frustration at failing to find the right man.

"True happiness seems so impossible!!!!!" she tweeted earlier today, before referencing one of her songs. "Maybe I'm just CALIFORNIA DREAMING!!!!!!!!!! (sic)"

Rihanna recently revealed one of her New Year's resolutions is to find a man, but she has admitted her love life is "non-existent" because her career is so demanding.

It has been reported recently that Rihanna has been exchanging secret Twitter messages with her ex-boyfriend Chris Brown, leading to speculation of a reunion.

Rihanna went on to post another message where she appeared to be considering the fragility of human existence.

"Life is short...TOO short!!! (sic) she wrote.

The 23-year-old star has clearly struggled to unwind since the end of her Loud world tour.

Record bosses allowed her to extend her trip to Barbados so she could catch up with friends and family and recharge her batteries.

However, earlier this week Rihanna admitted she was finding it hard to sleep.

"Waited all yr+Finally I have time off, time 4 rest n quiet.Suddenly all the silence is being drowned by my thoughts! No sleep #HEAVY #SCARY (sic)," Rihanna wrote on her Twitter page.

Meanwhile, Rihanna has been named the biggest-selling digital artist of all time.

The star beat chart rivals such as Lady Gaga and Katy Perry to claim the title after selling 47.5 million digital downloads during her career so far.

The Black Eyed Peas claimed the runner-up spot with sales of 42.4 million, while Emimen was in third place with 42.2 million downloads.

Lady Gaga and Taylor Swift rounded out the top five, while Lil Wayne, Katy Perry, Kanye West and Britney Spears all feature in the top ten. (C) Cover Media