It's great that you're sharing such personal information about yourself and reaching out
You mention taking antidepressants, so do you see a psychiatrist? It's important that you see one (if you're not) and find a dosage that works well for you and helps you not feel so low so often.
Do you see a therapist/psychologist/counselor? Some are even especially trained to deal with self injury, and most who deal with "mood disorders," which includes depression, (should) know how to handle patients who self injure as well.
It's agreed upon that if you were to hurt yourself intentionally you are most likely in an extreme amount of pain and/or are having immense difficulty handling and coping with intense emotions.
I take antidepressants too, and I have both a psychiatrist and therapist. I've self injured by hitting myself too, and though I haven't done so in a little while, I have recently thought of it. I try hard to share that with my therapist because I honestly can't share with anyone else. Even if I could, they wouldn't know how to help me at all.
Even though you've been struggling with self injuring for 5+ years, it doesn't mean you're weak or crazy or anything. It seems like you simply (and unfortunately) have not been able to get the resources you need that would help you cope with any problems you may have and deal with your painful and intense emotions in much healthier ways.
I hope that was helpful :-/
And, also, I tried cutting once and IT HURT LIKE HELL!!! Seriously, I don't know if you have already cut or not but if you haven't, just know it is, like, 10x's worse than hitting AND it takes FOREVER for the cut to heal (and it may leave a scar).