Limit dropdown list based on selection from another list

Let me preface my question with the fact that my Access database knowledge is very limited. The database I'm using is an exceedingly simple set-up where I enter the Date, Hours, Type, SubCat, and a Memo into a table to record my daily time. It's all in one table and I've used combo boxes to create drop down lists for the Type and SubCat. See attached image to get a visual on what I'm referring to.

A new year brings new categories and a need to change this from a two-level structure to a three-level structure. Previously using just "Type" and "SubCat", I now need to add a level between the two labeled "Category" (which I have already done). The functionaility that I didn't previously have (or need) is the ability to limit the list selection based on what is previously selected. For example, if under "Category" I were to select PIT, then I would only want Help Desk, Solutions, DNN & TTCs to be my dropdown options under SubCategory. I would like to customize each Category with it's own subcategory listing. (Please note that it is possible that a Subcategory may be an option under more than one Category.) The attached Excel document is my "map" as to what options should be available once a prior selection is made. I hope that this is making sense!

Thanks for any help that can be provided. My hope is that I can just change a few settings or make some minor alterations to my existing database to gain this functionality.

Wow, thanks for the quick response! I got as far as creating the new Categories table and adding all the possible combinations as separate records. You've lost me at the next step though because you're referring to a form I think. I don't have any forms. Just the table that you saw a screen shot of. I attached a copy of the database (figured out I could do that if I zipped it) so that you can see what I do/don't have going on. I'm certain you're heading me down the right path and I'm probably just overlooking what I need to do next.....

Thanks. I'll eventually figure it out. We can't get Excel to do what we want either and have been using this simple database for a couple of years now very successfully. It may be that I have to resort to creating a form for time entries, but that was something we were hoping to avoid. Thanks anyway!