Need to know the following:
1. What exact Mac OS are you running? Also, how did you "arrive" at that version? Did you upgrade "in place"?
2. Have you ever done an disk cleanup/maintenance/repairs, from a software perspective? By that I mean have you ever run any disk-related software to help with those tasks? There are some excellent products available (both free and commercial) that can help you with that.
3. This is critical: are you making backups to an external device? If you are, what software are you using for that?
4. How much free space do you have on the SSD?