Why do people with bipolar disorder have such a hard time letting go?

even though it isn't thier fault, why is this?

I am not so sure how I can answer you question without going into boring details.

What I can say though is the need to refrain from asking why, because there is really no answer. It is really nice to know that you recognise the fact that it is not their fault. Generally, what people with mental illnesses, like bipolar disorder, really need are supportive family members and friends - not easy, as it would be a constant learning process for all.

no matter what they have done, it came from part of them. It's not like you don't feel bad when you do something stupid when you get drunk... it's not that simple, girl
Think about this.

A lot of people without bipolar disorders have a hard time letting go. What more people with a chemical imbalance that triggers depression and mania.
some are delusional and they hang on to ever word wanting to believe in that person but they also can turn in a second to hate once this person has been hurt to often. They are very sensitive people and carry everything true to there heart. Don't piss them off or back them into a corner it can be dangerous. It just takes them a little more time.
Bipolar
It is part of being bi-polar. Unless you understand the true meaning of it there is really no explanation.
WELL I'M BIPOLAR I'M 19. LETING GO OF ANYTHING AND EVERY THING IS HARD.FOR ME I GET IN THAT FU**ING BIPOLAR MOOD I THINK THE WORLD HATES ME I'M NOTHING.I THINK THAT SH*T STAYS WITH YOU FOREVER.LIKE IVE SADE BE FOR IF YOURE NOT BIPOLAR YOU WILL NEVER REALY GET IT......