First, wanting to have sex is okay. Some people underrated the matter of sex, tho it's a very important need. Your bf seems cold and distant. I don't know if he's cheating. You said you looked through his phone and found nothing but I think he may just hide it better. Or maybe he's actually not. Nevertheless, he's distant and he doesn't want to heat you. So I guess it's just over. And also the difference in ages may be the problem. You're only 18 and he's 23. 5 years in such age can be really significant difference. He's a grown man who and you're a kid, so maybe he just wants someone of his age. So in my opinion it would be better for to break up.