Deleting partitions and/or formatting isn't guaranteed to actually delete the contents of files. All that these utilities do is to rewrite various file system tables. Raw data written to disk sectors can still be read and reassembled.
There are delete utilities that will actually overwrite the sectors used by a file with random data before releasing them as free to re-use. But based on the way file systems work, older revisions of your files may have written to sectors that the file system has lost track of in the course of new writes. There is a risk that this stuff will stay around.
The only way to be sure is to do an entire disk "scrub", rewriting every sector with random data. Sometimes multiple times. My favorite utility is DBAN (Darik's Boot and Nuke). Basically a tiny Linux system, bootable off a USB drive or DVD with a menu to select disks, what and how to overwrite and how many repetitions. The disks selected are all mounted as data drives. There are no problems with trying to use the OS to delete itself, as that is running off the USB drive.
https://sourceforge.net/projects/dban/
Some caveats: The utility is not recommended for SSDs. The original DBAN is targeted at x86 systems (old 32 bit). The above page references a newer, maintained utility called Nwipe/ShredOS:
https://sourceforge.net/projects/shredos.mirror/