<human>: Summarize the given text in a few sentences.

I'm a guy, almost 20 years old, have been in this relationship for 19 months with a girl i've known for 4 years who is similar age. I love her and she loves me.

BUT, I'm scared that although I do love her, if this relationship doesn't end we could end up marrying and I will always have this thought in my head, what if I could have loved someone else more. I want be sure that when i do get married I know that I could never love anyone more.

But how can I do this to my girlfriend? I mean the world to her. I'm not scared of the commitment; if after a few years of other relationships I find out you can't love anyone more I would try to get back with her. 

Is this a fair thing to do? I couldn't leave her hanging on to me if it was to be for a few years, but I'm not sure if she would get over me if I told her I might want to restart our relationship somewhere down the track.

So if I do decide to break up with her: How should I approach this? What can I say to her? How could/will she react?
<bot>: I want to break up with a girl I love to see if I can love other people as much or even more, to make sure that if I marry her I know I've made the right decision. Is this fair?