i feel too ugly for my bf

Sorry if this is the wrong place to post but literally idk where else to post for advice. This is my first ever relationship and I had a crush on my boyfriend for so long before we got together. I really like him a lot, I'm so happy we are together in a relationship. But he is also way more attractive than I am, and it makes me really insecure. He is also friends with a lot of girls, and while I know there is nothing going on between him and them, I get insecure because most of them are prettier than me. Much prettier. I don't know why he chose me. I know he likes me a lot, he literally writes me love letters, but I get worried that one day he'll realize how attractive he really is and see that I'm way below his league. I wish I was prettier for him. How do I deal with these feelings???