How to present filtering items in a sidebar?

I have a site where I need to present filtering items in a sidebar. These items work as "filters" for the site's content on the right. The user can click these filters on or off (selected are highlighted), and the content on the right side populates according to these selections. The problem is that there can be even 40-50 of these items in this view in multiple different groups and the user needs to be able to see all of them. How do I represent these items in a clear and understandable way?

The picture is a sketch of the current situation that the items are in now. Every item has a recognizable icon attached to them, which helps identifying which filter means what. Could the items be shown only as icons to save space, or is that a bad pattern? There isn't much space in the sidebar as we need to include also other tools in there (settings etc.) which adds to the problem.