So, here's the thing...

What are we talking about?
Oh, yes...
Loss aversion, the correlation to relationship failure and the delay of the inevitable.

Well there is a fundamental flaw in the statement/question.
You are not loss adverse about relationships.
You do not fear the person leaving you.
You do not fear losing the person.
Both are completely inaccurate.
You fear that you you will be truly happy without them.
You fear that because of the aforementioned assertion that the time you spent was ultimately useless... a waste of time.

Ego.

Humans are communal creatures.
At the extremes, herd-like...
It is not the fear of our circle shrinking.
It is the fear that we do not need the kind of social interaction we have been told we do.
It is not that we mourn the loss of community, instead that we truly never need the community to begin with...

Let's take this where it needs to go.

You are afraid to truly, deeply, madly love yourself.

In everything that you have done, are doing and will do.
You are afraid to unapologetically love yourself.
So, you pour love into others hoping to get them to love you the way that you need to complete you.

You know what?

You will never get it back the way you need it.
The way you are designed to receive it, because that is your job.
Passionately love yourself... First.
Then nobody is a need in your life.
You don't require anybody in your life.
Then you truly give your life to some else.
That is the gift.

That is the fear and loathing of love and loving others.

So here is the thing...

We do not fear the loss of the relationship, we fear that the entirety of time, emotion and pouring into it was meaningless... without definition. This is loss aversion.