What is the process for cleansing a very negative action? Several years ago I did something that was totally legal, but very much against my upbringing, my world-view, and my ethics. It was a spur of the moment thing and I have regretted it massively ever since. However, it has haunted me mentally (and to an extent physically, due to tension) ever since.

I might add, this event didn't hurt anyone else.

What can I do from a Buddhist perspective, to completely cancel out this action and its resultant negative thought-stream once and for all?
Nyom Eugene,

it's most needed to speak with an "admirable" friend", a teacher (one who actually knows), at least not only to know if it was a fault and which kind of. As your words here give much doubt in regard of whether it even wars or if "you world-view" has something to do with what is right or wrong, and since it is not a proper enviroment here, it's good when Nyom seeks for refuge at proper place.

All my person can offer are proper places here. Amends &amp; confession or Uposatha/Sila ceremonies. 

The pattern to come out of a fault is simple: recognize it clear as fault, be ashamed and express it, firm resolving not to ever engage into such again. That's it. Clean. Yet of course does not make deeds undone, but when effects ripe later, easy to bear and one can after cleaning move on, having been lifted out of the hole.