Welcome to PC.

When the pain inside gets greater than what someone is prepared to contend with then SI may be the only coping mechanism they have found available to them.

They are really lucky that they have someone who is as concerned as you are and trying to be supportive. Hopefully you will come to where you are able to support them without causing yourself distress. This has nothing to do with someone wanting to cause you pain but may be the only way they know how to deal with their feelings. I hope both of you remain safe.