Sometimes doing teshuva just means regretting what we did and wishing we hadn’t done it, and taking on ourselves to try not to repeat the same mistakes (as they present themselves on our current level according to who we are now), but we can’t actually erase the past and we can’t necessarily erase its effects in a visible way that can be seen in this world, even though we may be able to change their effects on our Neshama.
For example, if Reuven murders Shimon and does complete Teshuva, Shimon will still be dead. If Reuven doesn’t send his son to Yeshiva and does Teshuva for it when his son is 80 years old, his son will still be an am haaretz.
If someone doesn’t learn the way he should and does teshuva at the age of 80, he will probably never become a Talmid Chacham.
That is why it is a complete chesed from Hashem that He enables us to do teshuva even though it is really irrational that it should be possible to do teshuva (as the Gemara says).