Ejaculate from kissing

I (18M) have a big problem. Almost every time my girlfriend and me kiss passionately I have an orgasm. It’s mostly when I’m in bed and on top of her or she’s on top of me. It’s really a burden for me because it gets in our way when I want to kiss for a longer time, and afterwards I’m not ready to have any real sex so it’s disappointing for both of us. Is this normal at my age or should I become more concerned? Does anyone know a solution?