"This Is Really Happening" is a song by country music artist Britni Hoover, which was co-written by Blu Sanders and Taylor while she was signed with Sony.  Taylor sold the song to to Britni Hoover, who later put it on her 2007 album "Country Strong".

BACKGROUND

Blu Sanders has a first hand account of writing the song with a young Taylor Swift:

"If I remember correctly, she came in with the first couple of lines which I thought were some pretty heavy lyrics for a 14-year-old," Blu recalled incredulously. “I was a bit intimidated, frankly. Guy in mid-thirties with confident teenage co-writer. People like to bash young, successful artists. Attribute it to their label, marketing, the target market, but Taylor was the real deal. I know first-hand that the girl at a young age, before her first record ever came out, was more driven and focused than I’ve probably ever been. When I was 15, I was kicking the soccer ball around, not writing songs!”

-- Blu Sanders

Despite their differences in age, they soon found a winning formula with which to work together. Taylor offered the first few lyrics of the track, while Blu began to experiment with some alternative tunings and the pair also collaborated on suitable melodies until eventually the framework of a song came together. However, Taylor knew what she wanted in some regards from the outset. “I remember throwing chorus ideas to her and she was certainly not afraid to say no to things she didn’t like,” Blu explained.

When the track took shape, it was called "Is This Really Happening?" and it became an ode to a love affair that seemed almost too good to be true. “It’s about being in a relationship or a moment with someone and it’s amazing and you sort of have to pinch yourself and wonder if it could be as good as it seems,” he added.

However, while Blu connected instantly to her as a writer, he wasn’t sure how to relate the teenage optimism that flowed through her work. “The challenge in writing with someone else is making [the lyrics] mean something to two different people with completely different lives,” he revealed. “I’m a jaded 30-something. If I had to guess, I’d say a teenager has more of those than I do. Maybe I need to rethink my approach!”

Despite spending several hours on the track, it was never recorded in the studio with a band – instead it seemed destined to remain an acoustic demo forever. The irony was that, just like the song Taylor had been picturing, the love affair was more of a fantasy than a reality and never really got off the ground.

“I still think the songs stands up on its own,” Blu declared. “I love the lyrics and I love the melody and I actually thought it was going to make her first album. But then I’d be [speaking to you] from my beach house, instead of my brother’s air mattress, if it did.”

Source: "Taylor Swift: The Rise Of The Nashville Teen" by Chloe Govan (Omnibus Press, 2012)