macOS Catalina not allowing me to access full storage on my disc
I fixed this by doing the following:

I ran  diskutil list. Then from that list, I checked which part of my disk was the one apple was using in the GUI in my original question by checking which one's size was also 42.74 GB (Its identifier was disk0s2 in my case).

I ran sudo diskutil apfs resizeContainer [the identifier we retrieved above] [the size we want it to be, so 87G]


2.5 In case this does not work the first time, run diskutil repairDisk and try again.