I think it is common for all of us to search for answers when we go through a separation/divorce. I am sure it is more common for people who didn't ask for it to happen. I can't really speak to the other side of things. It is a constant search for the reasons why your former partner left. Everyone I have talked to has done it, and we seem to wrack our brains looking for everything that contributed to them leaving.
Was it that one thing I did? Could I have avoided it? If only I would have done that one thing differently when they asked. I have spent so many hours thinking of these things. Trying to understand, trying to know what I missed, everything that contributed to it. It often seems like it is just one thing or a couple of things that contributed, but it's not. There are often dozens of reasons, many of them internal and a reflection of their view of the world, which we can never fully understand.
There is some value in searching, it can help healing and it can help us ensure that we don't make the same mistakes in the future. But there is a time when it must end. We must all realize that there are just things we will never ever know. This is a hard thing to do, but it has to be done. We must learn our lessons and then just try to move on. We must accept that there are things we will never know and somehow find a way to be ok with this fact.
When it comes right down to it, there are so many things going on in that other person's mind that we can never understand, most likely there are things that they never even mentioned. Sure, they may have told us how bad they think we are, or what they have concluded, but there are so many inner thoughts and even their insecurities that played a part. They made a choice and we have to live with it. The truth may never be known and there is nothing we can do about it. It then goes back to something I always write- we must focus on ourselves and making our lives as positive and enjoyable as possible. That is the only thing we can control, that is the only thing we can do.
I have had times when I am better at this and times when I am worse at it. I have worked through never knowing, only to fall back into searching for answers. I can tell you that I am much more at peace when I accept it than when I am trying to 'figure things out.' I encourage everyone out there to try your best to just let go and not get caught in this pattern of thinking. I know it is easier said than done.