I know how hard it is - been there often enough but no is necessary if you have any hope of remaining a sane, functioning, rational adult.
Not everyone is entitled to your time and energy. So learn to say no - and mean it.
You reach a point of accepting that you made mistakes,
they cost you dearly and that you can't change the past.
At that point you stop raging against your history,
and move forward
into a calmer, less self destructive frame of mind.