[18/M] Why does it feel like my girlfriend doesn't want to have sex with me?

Me and my girlfriend have been together since we were 15 and now we are both 18; so we've been together for a little more than 3 years. We started haveing sex after about a year, and then we had it almost everyday. Nowadays, though, I feel like she doesn't want it as much as she used to. It's not that I think that she doesn't love me, because I know she does. It's just that it feels like I'm always the one to initiate, I always want to talk about what I'd like to try and I always tell her how much I want to have sex with her. I talked to her about it and she said it's because she's so stressed (hard time in school) and that she feels that I'm putting pressure on her. But I love her! And I want her to get the same wonderful pleasure as I do when we have sex. Is it my fault? Am I just too much into it? Or is it because of her and my age? She told me that she had heard that women usually get hornier later in their life. Or is it because I'm doing something wrong? Isn't she enjoying it as much as I do?

tl;dr Why does it feel like my girlfriend doesn't enjoy sex as much as she used to? Thanks in advance!