next_post_link() accepts a third argument which will limit the links to the same category. There is an example of this usage in the Codex:
next_post_link( '%link', 'Next post in category', TRUE );
- in_same_term
- (boolean) (optional) Indicates whether
next post must be within the same taxonomy term as the current post.
If set to ‘true’, only posts from the current taxonomy term
will be displayed. If the post is in both the parent and subcategory,
or more than one term, the next post link will lead to the next post
in any of those terms.
- true
- false
Default: false
What you need, in short, is to add , TRUE to you function arguments:
next_post_link( '<div class="nav-next">%link</div>', 'Next project', TRUE );
And:
previous_post_link( '<div class="nav-previous">%link</div>', 'Previous project', True );