What is the process for cleansing a very negative action?

The Buddha taught every negative action arises from "the element of ignorance" rather than is performed by "the self". "You" did not perform the negative action. What performed the negative action was "the element of ignorance". This is the path of cleansing of Noble Practitioners.

Refer to suttas SN 45.1, SN 12.17, Dhp 172 & 173 & MN 115.

I'm not a professional or anything like that. I don't have a bunch of garbage to text. Simply forgive yourself.

According to SN 7.21 (quoted below), one could purify oneself of sins, not by water purification (as practised by Brahmins), but rather by dipping oneself in the lake of the Dhamma (Buddha's teachings) which has the shore of virtues (sila) - in order words, by practising the teachings, we can purify ourselves.

The Buddha said to Sangarava:

“Is it really true, brahmin, that you practice purification by water, believing in purification by water; that you live committed to the practice of immersing yourself in water at dawn and dusk?”

“Yes, Master Gotama.”

“But brahmin, for what reason do you practice purification by water?”

“It’s because, Master Gotama, whatever bad deeds I’ve done during the day I wash off by bathing at dusk; and whatever bad deeds I’ve done during the night, I wash off by bathing at dawn. That’s the reason why I practice purification by water.”

“The teaching is a lake with shores of ethics, brahmin,
unclouded, praised by the fine to the good.
There the knowledge-masters go to bathe,
and cross to the far shore without getting wet.”

When he had spoken, Saṅgārava said to the Buddha:

“Excellent, Master Gotama! Excellent! … From this day forth, may Master Gotama remember me as a lay follower who has gone for refuge for life.”

According to the Salt Crystal Sutta (quoted below), the notion of purification by practising the teachings is expounded further, through the explanation that one should be "developed in [contemplating] the body, developed in virtue, developed in mind, developed in discernment: unrestricted, large-hearted, dwelling with the immeasurable". If you dive deep into what this phrase means, that's all you need to cleanse your negative past completely. You can find some useful commentary by Piya Tan here.

"Suppose that a man were to drop a salt crystal into a small amount of water in a cup. What do you think? Would the water in the cup become salty because of the salt crystal, and unfit to drink?"

"Yes, lord. Why is that? There being only a small amount of water in the cup, it would become salty because of the salt crystal, and unfit to drink."

"Now suppose that a man were to drop a salt crystal into the River Ganges. What do you think? Would the water in the River Ganges become salty because of the salt crystal, and unfit to drink?"

"No, lord. Why is that? There being a great mass of water in the River Ganges, it would not become salty because of the salt crystal or unfit to drink."

"In the same way, there is the case where a trifling evil deed done by one individual [the first] takes him to hell; and there is the case where the very same sort of trifling deed done by the other individual is experienced in the here & now, and for the most part barely appears for a moment.

'Now, a trifling evil act done by what sort of individual takes him to hell? There is the case where a certain individual is undeveloped in the body, undeveloped in virtue, undeveloped in mind [i.e., painful feelings can invade the mind and stay there], undeveloped in discernment: restricted, small-hearted, dwelling with suffering. A trifling evil act done by this sort of individual takes him to hell.

'Now, a trifling evil act done by what sort of individual is experienced in the here & now, and for the most part barely appears for a moment? There is the case where a certain individual is developed in the body, developed in virtue, developed in mind [i.e., painful feelings cannot invade the mind and stay there], developed in discernment: unrestricted, large-hearted, dwelling with the immeasurable. A trifling evil act done by this sort of individual is experienced in the here & now, and for the most part barely appears for a moment.

The teachings on the Brahmaviharas in particular are further elaborated in AN 10.219.

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 & 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.