How come i’m no longer horny after one orgasm?

I’m a 21 F and my bf is 23. He is the only sexual partner i had and I generally enjoy what we do a lot. I just have one issue—an issue I’ve always had, now that I think about it. I hear about women having multiple orgasms, but I find it difficult to continue sexual activity after I’ve had an orgasm. Whether it’s masturbation, digital stimulation from a partner, or regular sex, once I finish, I’m out. My partner always wants to go for another round and i do too! But just can’t because i’m not horny anymore. Is this normal? And what can I do about it?