I'm finding myself in the same situation as my friend.
My gf doesn't want sex as often as I do, and she's not willing to do/try some things I really like in bed despite promising she would try. However, whenever I bring up the promise she made, she just says "I don't wanna be pressured". Yeah, right, it's been almost a year since you made that promise.
Relationship's been great otherwise but this single aspect makes me contemplate getting my sex from somewhere else and/or breaking up with her.
Like the saying goes, sex is 5% of the happiness in a relationship but 50% of the unhappiness.