From what i understand so far about his healing abilities it doesn't affect memory, also note that i did not say life forever just that he never age.
With his elemental sight he can read edios up to 1 day proir and nothing more.
After that he use regrowth simply to change the body back to how it was then and his memories doesn't reset (After he healed his eardrums nothing changed).
Was it not Sakurai who died from overuse of all her psions and that was the reason not even his magic was able to keep her alive.
The Theory its very simple:
Say Tats use the 24 hrs time frame available to restore the object, in this case the person body, so he start today at 00:00:00, tomorrow will be at 00:03:00, after tomorrow at 00:06:00 and keep going, the 3 seconds variable its cuz error margin between Magic Restore type is summon and late applied, could be 5 secs or less, each advancing day, Tats needs to summon/use restore 3 to 5 seconds before the time frame change, I think everybody knows the hour 24 doesn't really exists, the clocks change from 23:59:59 to 00:00:00, so if Tats doing this every day, instead of being a day older, you become 3 to 5 seconds older, do the math how long will take to your body reach 70 years old, means: Body age vs World time, so its not "Immortality" its more like you get old slower than the rest, something happens in Real World, take as example Turtle vs Mice.
About Life force, well almost all Religions accept the body its a vessel for spirit, conscience, life force or body energy, depend on the mith or religion you could fit the term, basically speaking, we die by age cuz the body degradates to no reconstruction point, common human DNA have certain point which cant continue their work, far I remember, we "change" our entire body like 3 times in normal life expectancy, due cells restoring process, there's its the point age research scientific s are working hard, to see if they can modify or change those cells which used their reconstruction pairs to certain point, like I said: like turtles, its was found types of cancer and sickness which aging the person or give longer body age, but still working hard on that.
Personally I believe its depends on the DNA, the Person self, environment which give life expectancy higher or lower, my Grandpa have 105 years old and looks like (an act like) 70 years old person, go market, buy, sell, talk, all of that, recently he have problems but mostly its cuz biological issues, and from whom could guessing: hes was smoker, drinker, party lover, soldier, police, agriculture labor and time to time a playboy, he say not to the extremes but he enjoyed life, I see him and I think if you give him a new "body" he will continue more years, some persons you seem they just not have the impulse, even if have 40 years old (personally I see like "torch's" looks like from Shana no Shakugan).