So sorry to hear about this painful time in your life. You wanted this relationship to work for years, you two have shared positive memories, not just negative ones, and yet it's become clear to you that this can't go on. Really hard stuff.
Just the other day, I gave him an ultimatum that either he seeks treatment soon or we break up. After some waffling he said he would seek treatment, but he has barely talked to me since, saying he will never again tell me how he feels. Every attempt at a conversation has been met with indifference or anger.
I now realize that I do not want to be in this relationship, regardless of whether or not he seeks treatment.
Sounds like even with the guilt you feel, you also have a lot of clarity about your boundaries (rules for yourself), and it sounds like a rule for yourself is: I am able to be in relationships where my partner is actively in treatment; I don't choose to be in relationships with a partner with untreated mental health challenges.
Having a true boundary doesn't mean that life then feels great and we won't have difficult feelings like guilt. Having boundaries is often uncomfortable. There will still be conflict and problems, but boundaries protect us from the fallout.
I feel guilty for changing my mind about the engagement and the ultimatum. I feel I have made a messy situation messier. He has expressed that the past month has been extra difficult for his mental health, and I am worried that breaking up could push him over the edge. However, I don't think delaying the inevitable would be any better.
I have a big struggle with feeling like "it's never the right time" to talk about my feelings with others. I am a pro at coming up with reasons to delay difficult conversations. "The kids are with us later today, so I shouldn't say anything that will make my H distracted". "My H just told me he had a bad day at work, so maybe I'll tell him how I feel tomorrow, not today". "H just got a mean email from his kids' mom... there's no way I can also tell him that he did something hurtful to me".
We can always find a reason not to say something, or to delay saying something, when we are struggling with our own feelings.
That's not to say that there aren't truly bad times -- if a partner is going in for surgery and you have already offered to help with recovery for a set time... that's kind of the level of "OK, hold on for 10 days and then have the conversation". If you two are in a car crash, that's not the right time to have a breakup conversation, and that's an assessment based in reality.
That being said -- when BPD is in the mix, it may "feel" like it's never the right time, but that's different from "not the right time" based in reality (like the above examples).
Another thought is that it might be more of an announcement versus a conversation or dialog, because it's not like you're waffling and he might say something that would totally change your mind. Not that you won't listen to him for a set amount of time (I don't see a reason for a breakup announcement interaction to go on for 6 hours, for example...), but this isn't a "let's change each other's minds" conversation, fundamentally (as far as I can read in your post).
I am unsure of what to do right now. I am leaning towards having the conversation today. We have a trip planned for this weekend, which I think could be disastrous.
I'm in need of some advice and/or validation here. I don't want to cause him any more pain than necessary.
I am assuming that if you had the discussion today, you would not go on the trip with him this weekend?
Can you remind me if the two of you live together?
Do you have plans for moving out, or would you hope that he would move out?
He will be in pain because breakups are painful. He may struggle with that pain because of BPD traits. He may say it's the worst pain he's ever felt, and he may say or do dysfunctional things or make low-skill attempts to manage his pain. Are you concerned that he will threaten suicide?
What info would you want to share with him, where you would feel like you gave him helpful resources to manage his own pain (if he chooses to use those resources)? What plan would you want for yourself, for if/when you hear him say extreme statements about what he will do?
