OK - The first thing is not to listen to people on here who tell you to suck it up and masturbate, or conversely to people who try to give you tricks to make her respond to you.
The fact is, as you point out, that you are only 18. Sex is a hugely important part of your life as a young man. It is not separate from love. It is part of love. A marriage or a long relationship without enough sex is, for someone your age, is a disfunctional relationship, period. It is the very definition of incompatibility.
Why is your girlfriend sexually less interested now. That is hard to guess. But I will tell you that it is not uncommon in relationships. Sexual interest can decrease, in both partners, over a few years. In the beginning, the heat from the romance is so strong, that sexual drive is almost continuous. After a while, for one or both partners, other interests and anxieties can take over, sex can become less easy to access. For women, arousal is a key element of having sex. From what she says, your girlfriend is having trouble finding ways of getting aroused, now that the romantic heat of the first months has faded. This is HER problem. If she doesn't want to solve it, there is very little that you can do, no matter how much you love her.
Another issue is biological. Your sexual drive is at its height now in your late teens. Hers, or that of most women, is growing until her mid thirties. There is a certain incompatibility built into your bodies. In my teens, I almost always dated older women for that very reason. I was wildly sexual. So were they.
So what to do. If you two were older and you could handle it, I might suggest trying to open your sex to another male partner or two. That often makes the woman feel desirable and adds a lot of heat to your sex life, for both of you. But you have to be open to that kind of thing, and that may take living a few more years. I think, given your age, that you should take her lack of interest as a sign that things are not going to work well in the future. Resentments store up and turn into fights and coldness. You have lots of time to find other lovers, and better matches. It will hurt, but all growth does. It is most likely best to move on.
