Why have a long term relationship without marriage?

Hi everyone, I've been thinking a lot about my future lately. Im in a relationship with someone a few years older than I am, we've been going out for a year now and I think maybe some day we could end up getting married. But that scares the s*** out of me. She's my first real relationship and everything has went really well so far, but when I think about marriage and my future and that she might be the only person I'll be with for the rest of my life it puts a lot of pressure on how I feel about her right now. As in, if I don't really really feel deeply and madly in love with her every day I'm worried that I shouldn't commit any further, because in 5 or 6 years we mightn't like each other at all, and I keep hearing these things like, if you have any doubts at all that she's the one then she isn't but I think that's bs. I think you have to be an idiot not to think deeply about a commitment like marriage. But I know I'm over thinking, so to help me forget about marriage for the time being, what other reasons are there to be in a long term relationship if you don't intend on staying with that person forever? I'd like to concentrate on those reasons and not stare too deeply into the future.

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