I'm scared that my boyfriend will fall for or crush on another girl?

This might be a silly question but I'm feeling scared and insecure. I'm 18 and my boyfriend is 22
I've been through many failed relationships but this one seems to be different. For the first time, I've fallen head-over-heels for someone who fully reciprocates my feelings and has also fallen head-over-heels for me. He is so good to me, I'm not used to being treated so well. He texts and/or calls me every single day, misses me lots when I'm not around, compliments me a lot, actually listens to me everytime I talk, would do anything for me and protects me, etc. He thinks I'm too good for him and thinks he doesn't deserve me but really he's what I've always wanted in a boyfriend and I don't ever want to lose him. It's always been hard for me to trust guys but for some reason it's very easy for me to trust him. He's become like a best friend to me too.

I am scared that later on he will fall in love with another girl or even just have a strong crush/infatuation with another girl, my ex boyfriends have done that to me and I read and hear about it happening all the time. He said that he's "been around for a little while and knows its hard to find a girl like me" but I still wonder that when he's been with me for a long time and gets bored, if he will fall for someone else. Is the saying "if you love two people, choose the second person. Because if you really loved the first one, you wouldn't fall for the second one" true? And do all guys get intense crushes on other girls when they already have a girlfriend? I don't know why I'm worried about this lol I guess it just seems to happen in many relationships, it would crush me if it happened to me after trusting my boyfriend so much :'(
I'm scared that my boyfriend will fall for or crush on another girl?
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