tainted self is the tainted ego individual self
TRUE SELF is the eternal Buddha(the Buddha is eternal becuase all life is eternal(hence forever in samarasan being REBORN)
the Buddha is fixed because he dient change,he doent retrogress and become angry,or have ghreed he is PERMANANTLY ended these deflements,
so what u get is the Buddha has eternal life(as do we all)
BUT the Buddha is fixed and does NOT change as our tainted self does.
this is why the Buddha is calle the TRUE SELF:)eternal and never changeing(also all buddhas are of ONE MIND(lankavatara sutra)which is why u see some zen monks saying true self and ONE mind(dharmakaya)one in mind (nirmikaya)many in bodies(same enlightenemt)
I hope this helps this is Tathagatagarbha teachings (lotus sutra-----on the the last sutras