The normal course of bipolar can also cause neural problems - over time, our brains shrink faster/differently than the general population. So that probably isn't helping.

Some mood stabilizers (lithium in particular) have been shown to stop/improve this degradation. So one factor could be how long you've been on mood stabilizers, which would probably help prevent things from getting worse. If the mood stabilizers are recent, then it could be damage already done from years untreated - sad but true. I'm a little bitter about that one, having had my condition ignored by docs for 15 years!

But there's any number of things that could be going on - depending on age, there are natural things like hormonal changes that can really mess with cognitive performance.