Impact of the first love experience
This answer reflects my own thoughts on this topic, It isn't any generalized opinion of psychologists that I am putting forth.

First:- Each person is indispensable (not just the one we love), comes with his own characteristics, teaches us to look at the world from different perspective. But we notice the impact of the one we love more than any other because we want to feel how he looks at the world, how he feels. 

We want to feel how it is to be him? Therefore his characteristics become our characteristics (to an extent). Some people say that you become a person you are in love with. Now he has abandoned you, you miss him more and more because some part of him is still in you.

Failed love empties our world and induces a void within life. If given a chance to pinpoint the worst feeling related with failed love and longing for it, I would say that it's the vacuum that haunts us after the person we love doesn't accept us.

Therefore, trying to move on must be acknowledged as our attempt to fill that vacuum. Of course, by doing so we run the risk of being abandoned again. But that is small compared to our desire to fulfill our love, to fill that void.

Second:- Whether or not we look for someone who reminds us of the person we love is another paradigm of our love history.


How acrimonious the breakup was? 
What stage of love were we at when this happened? Because more the time, more difficult will be to give up on the one you love
Who according to us was responsible for the breakup?
How dedicated we were in that relationship?


If a person thinks that he was responsible for the breakup, definitely he would search for someone with similar characteristics and once he finds one, he'd be conscious this time and wouldn't try to repeat his mistakes in this new relationship.

If he holds the other person responsible for the breakup, then it is difficult to try to accept some other person with same characteristics.

Third:- Really, I can't comment on this third one.