A lot of opinions here!
I've lived with 10+ women. None of them went off sex. None however seemed to have the stable background your GF has. I'd bang them 3 times a day every day, but after a year, I went off it. That was a problem!
It's very unfair for other posters to suggest she needs to see a doctor! That just represents an infantile approach to dealing with stuff as an adult, using other so called professionals as in locus parentis.
If you smooch, cuddle & spoon - your relationship is fine! She likes sexual frequency less than you, but what's it like when you do? If it's good+ then that's another positive sign, & obviously a bit negative if it isn't.
Many women change their mindsets when you start to cohabit - she's probably thinking of the future, a career path, when to have a baby, & where to live - lifelong objectives - women do this all the time.
Men, including you, just think - when am I gonna get laid again!
So the mindset between just wanting to shag someone, & having an enduring relationship is completely different.
So, does she talk about the future - yours together? If so, & you love her, chill out - it's your hormones, not hers - just have a wank!
The only concerns I'd have, would be if she wasn't talking about & planning your future, in which case - you're a stop gap relationship for a serial monogamist.
A lot of guys feel an increased need to copulate with their partner when they feel that they may be cheating, or that they don't love them as much as they used to - I believe it's a throwback to the old - get her preggers & she'll stay.
If, as I suspect, all is fine however, is there anything else you could be doing? Going out with friends, study, a hobby you're neglecting?
You could be just too absorbed in her - you need to do things that you want to do too - outside of the bedroom. It sounds a little like she holds the power in this relationship, even if she acquires it passively.
Get out, have fun, don't be her poodle. You have to be a man, not a wuss. You may both find this rewarding, & if she doesn't like it, she'll either start feeling scared, & initiate more sex to get you back around her more, or she'll start bleating, & saying maybe she'll change, which you really should just ignore.
Either way, you'll be spiritually & emotionally more fulfilled. There's a world of wonder outside your bedroom walls!
Edit: Oh yeah, don't take people away on expensive trips in order to persuade them to have sex - it doesn't work. You only do that as a reward for having an amazing relationship, or to impress naughty new partners!