I have difficulty making decisions, but I also pretty stubbornly stick to the decisions I do make (even if they are irrational).
I measure my emotions in black and white. I am either "okay" (ie. not in immediate crisis) or "not-okay" (ie. on the verge or in full-blown breakdown). Anything else is too complicated for me, like how you said everything keeps changing. If I try to measure my emotions by "happiness" it ends up inconsistent because what makes me happy one day could piss me off the next. I know how exhausting it is to try to defend, justify, and explain yourself to others. I usually get defiant and turn it back on them, saying, "I don't question why you do the things you do, so stop!"
I summed up what you said in a sentence: "What I say is true, but how I feel may change." I hate feeling like I am always tricking others or that I am somehow being dishonest because what I say/do now is inconsistent with my behaviour later.
I think regret is a way we beat ourselves up emotionally. It is useful as a reminder to us not to make the smae mistake again, but I'm sure people like us play the incident(s) over in our mind repeatedly to the point where it is unhealthy.