did anyone break up with one they thought they were going to marry?

I am with my boyfriend and we’ve been together over a year and it still feels like we are in the honeymoon stage. we are great at communicating, want to be together all the time, work out our problems, are always there for each other, and have plans for the future. I can imagine marrying anyone else in the future. I know we’re young (21 years old) and haven’t been together really that long but I don’t know how i’d ever survive a breakup with him and find someone new to love. he’s my best friend. has anyone ever felt this way and ended up breaking up anyway? i’m just curious as I don’t know what could happen in my future - how did you get through it? kinda like i’m preparing myself to be devastatingly hurt :/