XT disc brake caliper with m486 levers?

FWIW. Presuming that there isn't actually a mechanical problem with your rear brake (i.e., there isn't grease on the pads AND you can clamp the brake caliper on the rotor enough to prevent the tire from rotating with the same amount of hand effort as you use with the front brake), I doubt that the actual problem is with the particular brake caliper AND the problem probably has more to do with your bike handling skills ...

The good news is that what you need to do to improve your bike's rear brake should only take you a few seconds to master ...

• simply put, you need to transfer your weight to the rear of the bike by shifting your weight rearward ...
  
• and, THAT is most easily accomplshed by straightening your arms & moving your butt off the rear of the saddle as much as the circumstance warrants ...
  

Find a parking lot or field (a grass field may be better the first time if you!?!) & PRACTICE by exaggerating extreme, panic stops, first, by riding at your commuting speed AND THEN coming to a complete stop ...

Then, experiment with moderating how much you need to get out of the saddle & shifting your weight to achieve the amount of combined stopping power that you want for different speeds ...

As when driving a car, anticipating the circumtances in front of you & around you will probably be a huge benefit toward lessening the stress you are experiencing with your commutes.