Why it hurts?

I've been asking myself why does it hurt.
Why does being cheated on hurt one so much?
Isn't the other person enjoying themselves?
Shouldn't I be happy for her?

As much as I may be, the sadness outweighs your happiness.
You feel replaceable, disposable.
Eventually because of someone being able to replace your love,
you start to fear.
Fear grows insecurity, and in time it breeds jealousy and all the other negative feelings.
You start to forget of the happier times and start working solutions on how to remedy the problem.
When all methods have failed, you start to wonder if the problem is you yourself.
Maybe you're just not good enough.
Nobody is perfect, nobody can give you everything.
Maybe I just lack that one thing.

They say all is fair in love and war.
Because you can use whatever underhand tactics you wish, and in war, everything goes.
Times like this I feel lonely, thats when I feel sad.
And I can't move on even if I want to,
because I stay true to my words.