Yes, this is part of bipolar. It's part of the symptoms which you don't find on the "diagnosis" chart currently, which is impairment to executive cognitive function, but more doctors are becoming aware, and research is being done all the time in this area. This is an observed problem with people with bipolar, and you are not alone. Like all the other symptoms, some people have it worse than others. I've also had a lot of cognitive decline and can no longer read books easily, which I used to love doing. Focus, memory, organization, and processing (absorbing information and understanding it,) are all executive cognitive functions which can be effected.

You're not alone in this.