I'm a bit of a romantic and believe when you meet the right person commitment isn't a problem. A friend of mine was a with this guy for five years and he finally proposed, then, before the wedding he met another woman and was married to her and had a child all in the first year.

People that commit to the wrong person sometimes just do it out of insecurity and boredom.
You have to ask yourself a few questions.

Would my be drastically altered if they left me?
Would I support them in anything they did, even if it took them away from me?
(I don't mean going to prison for murder, more a job that travels etc)
Is this person, in my opinion, the most desirable boy/girl in the world?

If you answer 'no' to any of them, it aint meant to be.
I met my husband thirteen years ago when I was fifteen, and I knew that was 'it' then, as much as I know it now.