How to keep page navigation within post's category?

I’ve searched and have not found a solution to this. I have a “previous” “next” page navigation bar on top of blog posts, but it seems to navigate only by date of post, rather than category and date. Is there a way to make it stay within the category that the current post is in?

I’d like to attach a screenshot to show exactly what I’m referring to, but I don’t see a way to do that. So, I’ve created a page within my blog that shows a jpeg with the navigation bar I’m referring to circled in red.

If you go to this page, you’ll see the exact page I used for the screenshot, and be able to use the navigation bar.

Of the items showing in the navigation bar, the one on the right is the same category as the post, but the one on the left takes you to a post that is in another category.

I know there’s got to be a simple solution for this. Unfortunately, the theme I’m using (Suffusion) doesn’t seem to have a setting for this. I’m pretty new to php coding, but I’ve gotten pretty good at using Firebug to figure out solutions, but not in this case.

Thanks for your help!