I can't readily agree that to show mercy is to act unjustly towards oneself.
... we may characterize mercy as the putative ethical value that justifies leniency in the infliction of punishment that is due in accordance with justice. Only someone who has cultivated a rational sensitivity to
this value in thought, feeling and action has the virtue of being
merciful. It follows that not every suspension or relaxation of a
just punishment is an instance of mercy; instead, leniency must
be shown for the right reasons. In the mercy tradition, these are
typically glossed as reasons of charity (or, equivalently for our
purposes, compassion or humanity) that have their source in the
value of relieving a wrongdoer's plight. These reasons reflect a
generalized love of humanity; in particular, a concern to alleviate
serious misfortunes that the human condition is prey to and
which, in consequence, one can regard as evils that could befall
oneself or those to whom one has special ties. (John Tasioulas, 'Mercy', Proceedings of the Aristotelian Society, New Series, Vol. 103 (2003), pp. 101-132: 101-2.)
In light of this, if I show mercy to someone who has acted unjustly towards me, I do not act unjustly (over again) against myself. That is to say that it is not the case that I add injustice against myself to the injustice perpetrated against me by the other person.
Rather, I am forgiving, I show compassion, charity, humanity, benevolence, to the other person. None of these responses is unjust either to the other or to myself. If I could exact just punishment, I do not act unjustly (against myself) if I suspend that punishment. I am simply motivated by other values than the desire to impose a just punishment.
On reflection I have doubts about the coherence of acting unjustly towards myself. Justice seems an interpersonal virtue, and injustice an interpersonal wrong.