My grandmother is a widow, she still talks about my late grandfather with love and affection, he was definitely the love of her life. Depends whether you believe it is possible to still love someone after they are gone, I guess. (I do).
Equally I don't believe all love has to be eternal to be 'real'. What's wrong with enjoying the moment?!
As to whether I believe in 'the one' I believe that people become that way over time if conditions are right. I will use a tree analogy if you will bear with me!
The twin trunks of a twisted bay tree wind around each other perfectly as a result of careful years of training, but each has space from each other and will remain standing if the other is cut down, but always keeping the memory (shape) of the partnership.
Other relationships are like divergent trunks - they grow steadily away from each other over time, and loss of one barely affects the other.
A third type of relationship would be like the ivy that grows up a large tree - it clings and has no strength of its own and will fall to the ground without the other partner, yet the tree is barely affected by the ivy.
In another type of relationship (co-dependent?) you could imagine two strands of ivy winding round each other - together they have a certain degree of strength but both parties are weakened and fall if the relationship fails, and do not keep their original shape/identity.
Sorry for the horticultural babble, hopefully someone will understand what I am trying to say!! 