Hi LotusBlossom11, glad you felt ready to join the group -- welcome
Am I following that you're basically at "100% done"? Like -- even if after you start wrapping up the relationship, he "sees the light" and "does a 180" in the moment, you would still be ending things?
Sounds like there are two strands to your question: one is the logistics, and one is more emotional.
Logistically, I'm curious about your living situation. Sorry if I missed this somewhere -- do the two of you own or rent your living space? If own, do you co-own it, or just one of you? If you rent, whose name is on the lease, and did you move in at the same time, or was one of you there first?
Does he work outside the home?
Is he in any kind of counseling or therapy, or was he in the past? Do you know the provider(s) name(s), if so?
How old is your son? And is your partner also your son's dad?
Emotionally/relationally, I'm curious about what your relationship with his sister is like. And what about his relationship with that sister -- do you think he trusts her?
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I think it's significant that you are able to depersonalize a lot of what's going on -- not that it doesn't hurt or impact you, more that you recognize that he is ill and it's not like he wants to have BPD. This is a tragedy for all of you together.
In terms of "passing off" supporting him from you to someone else, yeah, that is difficult. Understanding more about his history of therapy (if any) and his relationship with his sister will help. Another idea is to contact a suicide hotline, whether local or national, describe the pending situation, and get their feedback. They may have some insights or perspectives you hadn't thought of, that could help in decision making. I believe texting 988 (national suicide htline) works both in the USA and Canada.
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Do you think it is at all possible (even though you say not necessarily) that he would become violent towards you, your son, or your pets?