"When you were not here, they were, for me"

I wonder when people commit themselves into a relationship, by letting another person into their lives, does it mean they:-

a) Fill in the empty spot inside them that's always waiting to be completed?

or

b) sacrifice (ditch) an existing relationship and replace with new relationship?

Seriously if all these people would notice how much they have changed.
Or am I the one who needs to move on?

Yup. Move on is the only choice I am left with.