Hello, and welcome!
I have a MacMini (late 2012 model), but do not have it partitioned with a Windows partition. Need to ask you some questions:
1. What exact Mac Mini model do you have?
2. What exact Mac OS are you running?
3. Have you ever done any disk cleanup/maintenance/repairs? You actually can do a lot of disk cleanup on your own, and also there are some excellent programs available (both free and commercial) that can help you with those tasks.
4. This is critical: are you making backups to an external device? And if you are, what software are you using for that?
5. I suspect that your statement "I set it up with half of the memory in macOS and half in Windows" actually means that half of your internal drive is "set up" for your macOS, and the other half is for your Windows "system". Correct?
6. How did you set up your Windows "system"? That is, did you use Boot Camp, or are you using a product like Parallels?
7. How are you making posts here? Are you in your Windows "system" on the machine when you make the posts?
8. When you shut down the machine, and start it back up, does it start up within the Windows "partition"?
Once you answer those questions, we can help you better.