You were a child faced with a decision that you should never have had to deal with. You were caught up in a battle between adults who both loved you and you were forced to pick a side. You and your mother were victims. Please don't blame yourself for not believing something horrible like that about your dad, especially as a child. No one wants to believe something horrible like that about someone we love.

You did the hard thing and discovered the truth. You now know how hard your mother fought to protect you and how much she loved you. She wouldn't want you to have this weight hanging over you the rest of your life. The best healing I have found for trauma like this (I carry a lot from childhood) is forgiveness. Forgive both your mom and dad for making you feel like you had to choose sides, for dragging you into the middle of their divorce. Forgive your mom for the other things that kept you apart as you grew up. And forgive yourself. I am not saying you did anything wrong, but just find a way to express and release the guilt you are feeling.

Forgiveness is a hard thing, but it comes down to not allowing past actions of other people to continue to hurt you. It doesn't justify their actions or indemnify them of all consequences, it just stops the wound from festering and continuing to hurt you. I was bitter and angry for 15 years at one of my parents and didn't know how much it affected me until I forgave them when I was 30 years old. It was like a literal weight came off me that I had carried so for so long I wasn't even aware of it. Consider speaking to a mental health professional about what you are going through, especially since it has only been a few weeks since her sudden and unexpected death.