Why does editing a video by Adobe Premiere Pro more than once decreases its quality?
There is a big difference between


saving your project, then returning to the project to do some additional changes, such as brightness adjustments, then exporting a media file, and
exporting a media file from your project, importing the result into a new project, doing some additional changes, such as brightness adjustments, then exporting the result of that to create yet another media file.


In the case of (1), you are working with your original source media, which is pretty much as good as it gets in terms of input quality.  In the case of (2), if you export a compressed file (and virtually every playable media file is a compressed version of the originals, which may also be compressed), then you are working with a second generation of quality.  While you may not immediately perceive that the second generation file is not quite as good as your original masters, compression errors and noise accumulates every time through the cycle.

One way around this problem is to only ever export in uncompressed file formats.  But this is not practical, as uncompressed 1080p video is 2.98Gbps (compared that to your typical AVCHD codec that is limited to 28Mbps, about 100x less).

The practical way around this problem is to return to the project that created your first media export.  In that project, add the brightness correction you want as an additional correction, export using the original sources, and then you will have media quality that's as good as your original export, with the color quality you were hoping to achieve with your second export.