Scared to death of my future

I’m (M20) dating a girl (F21) and I love her very much, but we both feel as if we want to get married to each other (in the distant future down the road), but we feel that’s a big responsibility, and we don’t know if we’re really prepared to say yes to being with one person for the rest of our lives, I don’t know how to feel about it since we both talk about our future children and what we can do when we are married, I’m not sure what to say to her about it all.