How to deal with immense guilt over past acts of cruelty? Is sincere repentance and striving really enough?

Sadhu. It's fine now. Go on and keep the focus on the path toward liberation, not following other than that, good householder.

To endorse Dhamma Dhatu's answer: as a lay person living alongside regular people, a certain degree of skill is needed when approaching confessions and in some cases confession may not be the most appropriate approach at all.

Counsellors or psychotherapists can help you unpack the issue until you find yourself surrounded by many other satellite issues orbiting in your mind, thus you suffer more. I don't consider that helpful. Even the therapist has a strange compulsion towards suffering, but it's performed in a rather acceptable and sophisticated manner, so many therapist operate from an awareness that is stuck inside the global culture of suffering-based narratives. It has its uses, though.

With regard to your situation, I had found myself in a similar position in my practice, and I was not able to move forward without disclosing my actions directly to the person in question. However, it is possible to write about such things, and in those instances that can lead to a greater understanding and thus a release of some sort.

Crucially and simply, inner conflict is the way the mind keeps itself going. In doing this it generates time, space and causality, and it is within that threefold domain where you choose to hold yourself captive.

From a Buddhist perspective, time, space and causality is a succinct way of describing the five aggregates of clinging. As one progresses certain beliefs, views and perceptions fall away, and, out of fear, the mind searches for other artefacts so that it can continue to create inner conflict; in this instance, it is your old memories and the associated feelings.

Those memories are a type of thought form because the mind as a sense organ detects thought forms. What you think about those thought forms are your feeling/perceptions - these two are both combined as one. The feeling/perception creates a response - a volition or will of some sort - in your instance that response is more thoughts about how disgraceful you are, thus guilt and shame. That is the cycle; that is the inner conflict, and that is how we hold ourselves captive inside samsara.

The good news is that there is a way to drop out of this cycle. How that's done is by chipping away at these little stuck points. Sometimes it might seem arduous, but it'll be worth it.

Here is a good resource worth reading: https://manjushridharmacenter.org/rinpoches-wisdom/the-four-powers-of-purification-of-negative-karma/

The problem when we don't forgive ourselves for our past is many times we create a cycle where our self loathing will cause us to act in unvirtuous ways and we remain tied to our guilt, unconsciously acting in ways to perpetuate our current state and self image. As Carl Jung said: “Until you make the unconscious conscious, it will direct your life and you will call it fate.”

Most importantly don't feel afraid to seek professional help if necessary.

The Bodhisattva Vajrasattva may help you in your future meditations. OM VAJRASATTVA HUM

Higher Buddhism explains all things are "not-self" ("anatta"). In other words, there is no real self that is the "doer" of unwholesome actions. Instead, the Buddha explained (explicitly in SN 12.17) the "doer" of unwholesome actions is the element of ignorance. This is why a Buddha has infinite compassion because a Buddha does not conceive "self" or "agency" in beings. Instead, a Buddha only sees bodies & minds that act under the power of ignorance or, alternately, wisdom.

This said, for minds lacking wisdom, self-belief in self-agency can be very strong. Thus the Buddha said because of not understanding and not penetrating the Dhamma, this generation of people has become like a tangled skein, like a knotted ball of thread, like matted rushes and reeds. In simple terms, the mind can easily become overwhelmed by unwholesome actions performed. If the mind is overwhelmed by bad karma, confession with a wise person can be helpful. In MN 61, the Buddha taught his 7 year old son:

If, on reflection, you know that action it led to self-affliction, to the affliction of others, or to both; it was an unskillful action with painful consequences, painful results, then you should confess it, reveal it, lay it open to the Teacher or to a knowledgeable companion in the holy life. Having confessed it... you should exercise restraint in the future.

MN 61

Confession is a part of Buddhism. For example this search of Buddhist scriptures obtains 446 results for the word "confess". The following is a common phrase in the teachings of the Buddha:

Since you have seen your transgression as a transgression and make amends for it according to the Dhamma, we acknowledge it. For... this is growth in the discipline of the Noble One: that a person sees his transgression as a transgression, makes amends for it according to the Dhamma, and achieves restraint in the future.

DN 2

Give the nature of your question, if you are unable to drop 'self-blame' via accepting the "doer" of the past bad kamma was ignorance (rather than 'your self'), I recommend to find a Buddhist monk or wise Buddhist for confession.

It may be wise/prudent to not confess to family or even to a secular therapist due to their possible lack of compassion & wisdom.

What you did before was cruel to others. Assuming that you have ceased being cruel to others (which is not an assumption I make lightly, or take for granted, because 'ceasing' is far more difficult than most imagine), the question is this: are going to be cruel to yourself?

Don't bother. There are no reparations (reparations are entanglements in karma); there is no atonement (atonement is an indulgence of ego). There is only acknowledgement (this I did) and release (this I no longer do). Asking for more is self-satisfying.

If someone wants something from you because they see it as reparation, give it to them. That is their karma, and you should not add to it. If someone asks you to atone in some way, atone. It costs you nothing — or at least nothing meaningful — and eases their misery. If no one asks anything of you, rest assured you will inevitably find those who have been subject to the kind of suffering you have inflicted on others. That is your karma. Give of yourself to them as best you can.

Most people measure themselves by their misery, but the truth is that the infliction of misery is banal. Nothing you might have done to others is all that unique, or interesting, or peculiar. Karma is (by its nature) repetitive, ordinary, and timeworn. Whatever you might have done is merely an echo of things that others have done across the span of human existence. The only salve for the wounds of karma is to stop being an echo. Accomplish that, and the rest falls by the wayside.

The idea behind Christian confession is that the admission of weakness to another gives one strength to overcome that weakness. Honesty to another becomes honesty to oneself, and self-honesty is powerful. Confession to a therapist is extremely useful, because a therapist (by profession) won't judge you, and that will help with your own self-honesty. I recommend it. But beyond that... The world is complex beyond understanding, and simple solutions are generally self-serving. Don't be self-serving, because you will eventually discard the notion of the self, and realize you've served nothing.