What causes men to lose interest?

Why? Why do men act this way? I need answers. I'm seriously considering ending my relationship with my first love (M17) while I (F18) feel so unlovable, almost as if I'm begging for affection at this point. I took a gap year after high school, while he completed his senior year. When we were in school together, we instantly connected; we fell head over heels for each other, spending entire days wrapped in each other’s arms in the park, laughing until dark. It was pure magic. We even took the step of losing our virginity to one another, marking this as a special sexual relationship. For me, it was incredibly meaningful, but for him, sex seemed to be less significant. I believe this was influenced by his heavy consumption of porn, which may have contributed to feelings of disappointment towards me. I'm flat-chested, and I know his type leans towards curvy girls, which makes me feel inadequate. Even though he knew my body type, he often made comments that hurt, like, “I can’t wait for you to hit the gym and get a nice butt,” or “It’s okay that you have no boobs; you can still grow a nice bum.” I should have recognized these as red flags from the start, but I dismissed them, thinking he simply didn’t know how to communicate well since he had never had a girlfriend before. Nonetheless, these remarks made me insecure. I often found myself comparing myself to the girls at school I saw him notice. Still, he seemed to genuinely love me and always wanted to be around me. Now, I’m starting to wonder if his feelings were genuine or if I was just convenient for him. He would send me pictures of other girls with comments like, “Do your makeup like this” or “Wear this,” and I began changing my style and music preferences and using more makeup than I ever had just to please him. But it was never enough. I expressed how these comments made me feel, and he always insisted he was just “trying to help,” a notion I accepted at the time despite my doubts. Fast forward a year into our relationship, and I started to notice a shift. He wasn’t communicating like he used to; the late-night calls and all-day conversations faded away. I couldn’t shake the feeling that something was off and began questioning where the boy who loved me had gone. I started asking if he was cheating, why he wasn’t reassuring me, and why he didn’t seem interested in making me happy. He would redirect the blame onto me, saying I was starting arguments and that I was the one causing trouble, insisting I shouldn't worry because he loved me. Then, a month later, he revealed he wanted a break because he found other girls attractive. My heart shattered; it confirmed all my fears and doubts. The very next day, he changed his mind, promising he’d never leave me and that he loved me deeply. I wanted to believe him, but my trust was shaken, and my insecurity kept me from walking away. But now, it feels even worse. He seems completely uninterested, hardly calls anymore, and it's shocking how indifferent he can be when I would give anything for him, despite how he's treated me. I feel lost. If I lose him, I can't bear the thought of seeing him move on with the very girls I worried about throughout our relationship. I’m doubting if I’ll ever find love again, and I’m confused. Why was I attractive to him in the first place if I don’t fit his ideal type? I feel completely heartbroken. Please, I need advice.