Aesthetix29 said:
... why is it that they turn bitter...
Who gives a sh!t? It is what it is. Avoid her, build up social circles that don't involve her, and work on yourself until you get someone who is worthy of you.
Eventually the bitterness will melt away and she will only remember the good times.
P.S. Here is the technical answer to your question. We have layers of genes and mutations and evolution piled on top of each other, evolved over hundreds of millions of years. Somewhere in this jumble of interweaving genes is an vague instruction that says "if your relationship ends then act bitter". There is no reason for this instruction except that over millions of years women who followed it had a better reproductive success rate on average than women who didn't. But the key thing is that
they do not know why they are doing it and hence you will never get a logical answer if you ask them.