<human>: Can you succinctly summarize the text in 1-3 sentences?

Hi all, bit of advice needed if possible.

Been with my girlfriend nearly 5 years now (School + University), so quite a large portion of my life! She was my first everything in terms of relationships and things of that nature so I obviously have quite strong feelings towards her.

But what I can't get rid of is the little nagging feeling which is me questioning about how do I know if she's the best, most perfectly compatible possible person for me in the world? How do I know that one day if we decide to marry a few weeks later I wont just meet someone who I click with more, and avoid the feeling of regret? Obviously i have only met a very small portion of the worlds population now, and evidently always will. But still?!

I know this sounds like a very flippant thought, but it does concern me quite a lot. I've even had this chat with my girlfriend (should show how close we are??), and she say's that she doesn't know how, but she just doesn't think there is anyone she would ever love as much as she does me.

Is there any way to shake this feeling?!?! Does me feeling this way and having these thoughts mean there is no future for us?

Any help/similar experiences would be great.
<bot>: Been with girlfriend 5 years. Love her. But how do I know if she's the best possible person, the one i'm MEANT to be with?