My longest relationship was my first one- 3 years, 11 months- so basically four years. We broke up early this year mainly because we'd grown apart.
Starting a relationship at 14, but 18 you're pretty much a completely different person so things change.
My current relationship has been 6 months now, and it's totally different from the last. Even if you think you're going to be with the first person you fall in love with forever, it generally doesn't work out that way- not to say that it never does, because I've seen plenty of couples who've made it work- but wouldn't you much rather live in the present and be happy with everything day-by-day, instead of focusing on the future and missing everything in the mean time? What if something happens? But then again, that's just my opinion.
I learnt through my first relationship to take everything slowly and have fun, and enjoy it for as long as you possibly can.