I've been with my girlfriend for 13 years now and it's still great. We both don't want to marry and don't want childeren.
So nothing is really changing. We both like to travel, so it's convient we don't have kids. Feel kinda bad for my parents, but hey, it's to us how we want to live our lives.
I had some really good times as a single, and really good times with my girlfriend. Just choose the thing that makes you happy.