Here's a mental technique that I find useful for addressing many dour feelings, guilt among them:

When you're feeling guilty, it is sometimes helpful to close your eyes for a moment, re-open them, and pretend that you're a new homunculus.

A "homunculus" is a tiny representation of a human, and one classic fallacy when reasoning about how brains work is the homunculus fallacy, in which people imagine that "they" are a little homonculus inside their head looking at an image generated by their eyes.

It's an easy fiction to buy into, that you're a little person in your head that can move your hands and shape your mouth and that decides where to steer the body and so on. There is, of course, no homunculus inside your head (for if you are steered by a homunculus, then how is the homunculus steered?), but it can be quite fun to pretend that you are a homunculus sometimes, mostly because this allows you to occasionally pretend you're a new homunculus, fresh off the factory lines, and newly installed into this particular person.

Close your eyes, and pretend you're arriving in this body for the very first time. Open them and do some original seeing on this person you now are. Rub your hands together, look around, and take stock of your surroundings. Do some internal checks to figure out what this body values, to figure out what it is you're fighting for. Check the catalog of plans and upcoming actions. Check the backlog of memories and obligations.

There will probably be some housecleaning to do: homunculi are known to get a little careless as they age, and the old homunculus that you replaced probably let a bunch of useless tasks accumulate without realizing it. As a new homunculus you have the privilege of pruning the things that obviously need pruning. Maybe you'll look and say "Ah, yes, we're going to cancel lunch with that person; this body was secretly dreading it. I also see that this body is currently spending a lot of cycles feeling guilty about a date that went poorly last week; we can dismiss that, it's no longer useful for this homunculus. And also, "exercise" doesn't seem to be on today's schedule at all! How strange. This body definitely intended to exercise today; somehow it fell off the list. I'll put it back on."

It can be quite liberating to be a new homunculus, without any obligation to propagate the errors of the old one.

This is, in fact, a common technique for dealing with the sunk cost fallacy (also known as the "pretend you're a teleporting alien that just teleported into your body" technique). This is useful for avoiding sunk costs because the new homunculus has no reason to honor the old homunculus' sunk costs.

Say the old homunculus bought plane tickets which would let you travel to Texas tomorrow (and return in a week), and that the ticket is non-refundable. The old homunculus may well have an attachment to the "go to Texas" plan, and may try to convince themselves to go even when it becomes clear that the trip won't be worth the time. The new homunculus, however, has no such loyalty to the sunk costs: it can just evaluate whether or not to go on the trip regardless of how much the tickets costed.

This is also a technique that works quite well for managing guilt: it's often easy for the new homunculus to recognize lingering guilt as a bodily response marking malcontent about something that was done in the past, by the old homunculus. The best action for the new homunculus to take, usually, is to check what regretted action caused the guilt, check what pattern of behavior led to the regretted action, mark down a note about which cognitive pattern needs to be reprogrammed, and then dismiss the guilt (which has now served its purpose).

As a matter of fact, guilt and sunk cost fallacy are closely related: both are about suffering for costs that were paid in the past. The only difference is that guilt carries with it a lesson, an instruction to alter your environment and your mind so that similar actions don't occur in the future. With practice, it is possible to reflexively treat the initial gut-wrenching guilt as an instruction to update your behavioral patterns, and then dismiss the lingering guilt immediately. (Cognitive patterns, after all, take some time to train.)

In the interim I suggest pretending you're a new homunculus. If you start to feel guilt, then close your eyes and re-open them as a brand new homunculus. Notice the guilt, listen to the message it bears, and actually write down the behavioral pattern that you wish to change. Then spend five minutes (a full five minutes, by the clock) brainstorming ways that you might change the pattern and start retraining your mind. Then thank the guilt for carrying you this message, and dismiss it.

Eventually, this can become reflexive. Until then, I suggest occasionally becoming a new homunculus. In fact, I often use something like this myself, even though I've been immune to guilt for quite some time: it's a great way to see the world and yourself with fresh eyes, and that can be invaluable.