Is it normal to feel under appreciated?

Tastes will almost definitely be different.
Also her inexperience with guitar and music. A slow, steady song is something she probably likes and familiarises with. She can hear the notes, recognises beat from similar songs, and the repetitiveness provides a core to the song, so none of it's surprise.
Obviously, her reaction to, say, shredding won't be the same. She can't hear the notes, the lack of melody and quick sounds make it lack the 'support' and pattern of the slower song. It sounds like a jumble of noted, which she probably doesn't think you practiced or made up on the spot.
The slower song sounds longer and slower with more variation, so it sounds like it required more work than a quick, tense one-- which she won't feel the effort in.

I've offered to learn a song that is very guitar orientated but she says she doesn't think it's necessary because she likes how I play... (the boring monotonous shite) I'll try writing her a song that is challenging but rythmically appealing to her :)

Is she musically inclined? I agree with dappled, sometimes the only people who can appreciate 14 years of study are those who have walked the same path. To others, it's just boring. Have you written a song just for her? Simple or complex- I'm sure she would appreciate that type of devotion.

Maybe it takes 14 long years of learning to be able to appreciate 14 long years of learning. You naturally have a better understanding of what's tricky than she does. Or maybe your tastes are just different.

maybe she don't even like the guitar and she's just humoring you.

For me, under appreciated seems to be rooted in the following: I do and do for others, to a fault (Im working on this), then when I have to say no or simply wait il later, Everything that is wrong in their lives is suddenly my fault. I just smile, chuckle at human nature, and get back on life's beam.

Maybe she is just turned out by certien songs.