Pop-up modal - bottom sticky bar

No Idea how to describe the problem I have here but I’ll give it a shot:

In the process of creating a pop up modal for filtering options on my site. Currently in the process of designing the pop up that is fixed and takes up 80%ish of the screen. I then want to be able to add a div at the bottom of this menu to apply the filters (essentially just closing the menu), all of which I’m fine with. Except, the pop up modal has an inner scroll as its quite large, therefore when I set the apply div to absolute, when I scroll down, the div is stuck kind of half way.

Would really appreciate any help with this!

Here is my site Read-Only: LINK
(how to share your site Read-Only link)

Not sure if my share link worked there?

Here it is again just incase: