Before anything manifests in the material plane, it exists in the astral plane first.
Disturbances in the physical reality start out as irregularities in the astral body, which then become condensed into matter. This happens due to the repeated and habitual perpetuation of the energy that caused the disturbance.
Such as picking at an injury so it takes forever to heal completely.
When something that dramatically affects our nervous system happens, we have the natural ability to self regulate after the occurence. Such as physically shaking, which is a form of the body regulate itself after shock/trauma of any sort (which is why wild animals shake after stress, or why we shake after an intense orgasm).
What happens with us, rational animals, is that our mind gets in the way and instead of being present in the moment, going through the stress, then letting it go as it ends, we leave part of our consciousness in the past, as if the event is still happening. So every time we get triggered, the same "astral wound" gets activated.
With enough time and concentration, this wound densifies and materializes in the physical reality, as anything that negatively reflects that wound.
This happens a lot to wounds we don't consciously deal with, because they remain like an unattended open wound that gradually worsens until it eventually operates like metastasis (when cancer starts taking over the body).
This is where dreamwork plays a huge part. It is during dream time when we are able to fully experience our unconscious wounds and aspects of ourselves.
Dedicate yourself a few minutes a day to write and analyze your dreams and you'll avoid many negative wounds from materializing.
Remorse isn't supposed to make you feel bad, but to make you question why you aren't doing the things you think you should do (or doing things you don't think you should).
Every time someone passes away, there's always remorse of some kind, it's natural. Some will say "I wish I had done more for this person", others will say "I wish I hadn't done that to this person".
Whatever the case is, it's always about not following your truth.
We act with emotion, which always leads (eventually) to limited results. Emotion is limited. Based on the current state of your mind/ego.
If you are upset at someone and you think you should express that, then do it. But if you express that out of emotion, not alignment, it will always backfire. You'll lose them and regret having said those harsh words...Or, they'll leave and you'll wish you would've said what you wanted to.
When you act in alignment with truth, you will regret nothing. What was said and done was said and done, whatever happens to me or that person, I was in alignment with my truth. There is no remorse or regret.
This is true for everything in life, as people are always regretting "not having started to work out last month because if I had then I would have this result right now". Or when they are diagnosed with a disease and they wish they could go back in time and not eat so much crap. Others think differently, they think "I'll die anyway, might as well eat what I want".
Whatever the case may be, if you are not aligned, you will experience regret over your past actions.
You must be certain of what you stand for and your truth, otherwise you'll be like a dizzy cockroach in life, always running mindlessly from side to side, trying to escape or hide from something.
Restrain action if it is tainted with emotion. Practice mindful observation of your inner world, and this will come naturally at some point, your actions will always be aligned.