Sorry that you're going through that. I know the feeling. The thing I've come to realize (and I'm sure you know this already) is that there is no guarantee, no matter what you do, that your SO will stay. Look at marriage. People make vows in front of family and friends that they will stay with you forever - yet we all know that the divorce rate is through the roof. So promises, however well intended or how much they were meant in the moment, don't always last. Things change.
If you look at the pattern, then I would brace yourself for him to leave again. He's already attempted to leave twice and yet things haven't changed. It sounds like your BPD is acting up worse than ever, which drives people away.
If there's one thing I know, it's that this disorder is a self-fulfilling prophesy. If you are paranoid and afraid to lose someone, you will, ironically, drive them away. That has been my experience and it is harrowing to deal with. However, the flip side is that even if you are healthy and not afraid that he will leave, there's still a chance he will. Look at it like this: Your SO has been, up to this point, your emotional buoy - he validates you, he cares, he shows you attention, etc. You've clung to him and it's helped keep your head above water. But, your emotional storm is worsening and that buoy keeps drifting. You lose your grip here and there, but he's still there. But now, you're realizing you can't rely on that anymore. He's not the stalwart source of safety that you can cling to forever, because he might not always be there. You'll need to learn to swim. You need to be able to weather these emotional storms WITHOUT him.
For those of us with BPD, our relationships are EVERYTHING. They are our whole world. We put all our eggs in one basket. You can't keep doing that. I don't want to invalidate you or make you feel worse, but you said it yourself: you are in denial. Look at people's behavior NOT their words. Words are empty, promises are meaningless if the behavior doesn't align with them. The writing is on the wall and there's a good chance he will leave again, possibly for good this time.
Maybe the best thing for you is to take some time off from this relationship so you aren't burdening him. Focus on therapy and working through your feelings so you can be healthier to maintain a stable relationship AND be able to find happiness and fulfillment outside of a relationship. You can't rely on him to make you happy - people aren't built to sustain others. And I know from personal experience that, being BPD, I have drained people dry of their own happiness to help manage my own. And I hate myself for it every day.
This may not be what you want to hear. If it were me, I'd be looking for validation and reassurance that this relationship won't end. But no one, not even your SO, can give you that validation/reassurance. Maybe right now isn't a good time for a relationship - that doesn't mean you have to end everything. But this relationship needs to take a back seat to your mental health and I'm guessing he feels the same way. I hope that something I've said helps you. I'm sorry that you have to endure this pain right now, I know what it feels like and it's absolute agony.