You can make your girlfriend like sex, yes it is possible to "make" people like things.
There could be multitudinous reasons for her not wanting sex, not liking sex, or not wanting as much as you.
One thing you want to find out is what she like & dislikes. Do the things she likes & avoid the things she dislikes.
You mention she doesn't want it everynight, some girls (even some guys) when they hit the bed they automatically want to go to sleep.
If this is the problem then don't let her get into bed until you have had sex!
Before she goes to bed is she watching tv? on the computer? reading a book? if she is doing something like this then it doesn't matter. Just go up to her & do things she likes (like hug & kiss her) & work onto other things she might like (like licking her all over), by the time you have done plenty of foreplay she should be yours for doing anything you want, & if she isn't keep doing things she wants, its sex isn't it?.
If you come home when she is sleeping (say after working late?) and she is already in bed say to her you want to make more time when you can be with her (for doing mundane things like watching t.v. etc).
She is with you for a reason buddy, she loves you, just do things & don't give her a chance to say no, you need no reason nor excuse to go over & hug her or kiss her, just be enthusiastic and do your best to make sure she enjoys everything that you do. Let her continue whatever she is doing but make it so she can't concentrate.
make her want sex, make her feel loved, thats the way to go.
jeff92872 is right in that women don't want penetration as much as men,
unless they are that rare breed of woman that orgasms from penetrative sex.