I have less than 10GB free of 75GB in drive C and 68GB free of 195GB in drive D geometry dash
what I don't understand is after I use the internet for a while, the free space in drive C gradually runs out
The reality is that 10GB of free space in not nearly enough these days. As you browse the web, space will be taken up your browser's cache. You could free up some space on your C drive, by moving some files to your D drive. However, I presume you have just a single drive, that is portioned into the two separate drives. If is the case, I recommend temporarily moving everything on your D drive to an external drive. Then, you can delete the D partition and then extend the C partition to fill the entire drive. You can do that with Windows own Disk Management tool, as explained in the following link. If you have around 10GB free on your C partition and 68GB free on D, you would end up with just one drive, with about 78GB free.
There are tons of third-party partition managers for Windows, but did you know that Windows includes its own? Microsoft did a good job of hiding the Disk Management tool, but it’s there.
After doing that, you could copy any files you need that you backed up, to your C drive.