Just gloss through the seinen tag on anidb or whatever other listing site you want and read the premise. You'd probably find better stuff than what /wsr/ suggests that way. That said:
Episodes 4, 7, 8, 10 (not as pronounced but present), 17 and 18 of Mushishi all deal with this, The series is episodic so you don't need to watch the others, however, you still may want to watch the first 2 to get a grasp on certains aspects of mushi. Aku no Hana's MC deals with this a lot in the first few arcs but regret is not in and of itself the core theme. Incidentally, finding peace with oneself may be a part of it but it's moreso the authors attempt at an exposition of many social aspects in an autobiographical setting (i.e: genuinity or lack there of in individuals, the acceptance of the path set forth for one and the futility of resistance, the shallow motivations behind actions and the thinly veiled pretenses people give for them, the illusion of self percieved uniqueness, escapism, deciding between alluring yet superficial false appearances or unwelcome realites, etc). As with most adaptations aku no hana is better enjoyed with the manga though, and not neccesarily due to the artstyle choice. Abenobashi deals with moving on for it's last quarter. Death Parade had some of this for certain episodes, though it's been too long for me to remember how much. Again, too long, but I believe those are all the core themes to the movie colorful. 5cm per sec deals with that in regards to a past crush (some other anon rightfully summed it up as "tfw no gf: the movie"), it's pretty. Mind Game does a litte, but it's not neccesarily the focus. Tatami Galaxy does if I'm remembering properly.
