I have never inflicted injury to myself on purpose. But I don’t think all instances of self-injury constitute a grave matter. For example, in a treaty that is sealed by blood (the “blood compact”), where two parties cause a small wound to obtain blood, the act of self-injury would not constitute a grave matter.
However, I agree that all cases of self-injury, even slight ones, should be avoided. If this is something that a person does to himself frequently, then he needs help - professional help - because it can lead to something very serious.
In this discussion I am assuming that bodily mortification (which induces physical suffering in the spirit of penance), but which does not harm the body, is not considered “self-injury.” If, for example, fasting is considered self-injury, then even our Lord - who fasted for forty days in the desert - would be guilty of self-injury.