The ego resists change because it knows ultimate truth is constant, and therefore anything subject to change isn't ultimately real, and if the ego isn't ultimately real that means it has no power over you, the part of you that IS real and unchanging.
This is why the ego resists letting go of the past, because the ego is constructed entirely of the past -- your memories, your conditioning, your grievances, your desires -- everything that is specific to your past, which you alone experienced (and is therefore not ultimate truth because ultimate truth is universal to everyone and everything). Letting go of the past means the dissolution of the ego, and as long as you identify with the ego, it feels like YOUR death, too. But it isn't: once all falsehoods are gone, only truth remains, and your existence does not depend on illusions. Illusions don't sustain you; they trap you, until they're recognized as unreal.
Think of all the ways we try to pin our identities to something both constant and unique (because the ego desires to be separate and special because its survival depends on it). But something can't be both constant AND unique, because its uniqueness is defined and shaped by everything around it; the principle of being "dependently arisen," which encompasses the entire temporal, physical universe. The constant cannot change and therefore cannot be set apart, otherwise it wouldn't be universal anymore, and therefore wouldn't be ultimate truth.
The ego tries to attach a constant identity to features like the body (race, ability, gender, health, etc), intelligence, achievements, associations, and especially pernicious; past mistakes, failures, "sins," and shortcomings. But all these things are both temporary and particular, and therefore not ultimately true. They are temporary because they did not exist since the beginning of time, and they will all disappear with bodily death. Anything that is beholden to time is temporal by definition, inconstant, and therefore unreal. They are particular because nobody sees "you" the same way, and your opinion of yourself is not more credible than anyone else's (Whom do we have more misperceptions about besides ourselves? All concepts about ourselves and others are just beliefs, and we carry the most concepts about ourselves, therefore we carry the most untruths about ourselves).
So to survive, the ego carries forward everything past and tries to make it a permanent part of our identity: "I failed once" becomes "I will always fail, I am a failure." It doesn't matter if you failed 10,000 times in the past. It has no bearing on the present or future if you choose to not identify with past failure. The tree bears no resemblance to the seed, right?
This is the justification for universal forgiveness. Because in trying to punish past "sins" to prevent them from recurring in the future, we treat something temporal as if it's real and therefore constant, so our "crimes" forever make us criminals, never to be fully clean again, which doesn't make any sense. All "sins" stem from fear and self-hatred, which needs healing, not further punishment that will only exacerbate the pain. To forgive is to heal because it is letting go of the past & illusions and refusing to acknowledge them as real.
"There are no levels of difficulty to miracles," some past mistakes are not harder to let go of than others, because everything temporal is equally ephemeral. Nightmares and happy dreams are gone just as instantly when we wake up. Believing the nightmare is real is what gives it power over you. Everything here is a dream, and we're guaranteed to wake up eventually. Our identity does not rest on this planet.