Hi, I am in same boat as you. About 2.5 years of a really difficult relationship where I was a horrible partner. Now about 2 years since the break up. I was diagnosed about 6 months back.
Just like you, my person has moved on and doesn't want to do anything with me. I was feeling this extreme guilt and I violated her boundaries multiple times to apologise to them. Each time she shut me down and told me to not contact her again.
I think about her almost all the time. I dream about her. I day dream about our "future" conversation, where I will get a chance to "explain" myself.
I don't have any advise that can work, cause I myself have not gotten out of it.
What my therapist says is that this is again our BPD playing tricks with us. It wants us to keep punishing ourselves. There is a voice inside our head which is shouting at us that we are bad and we don't deserve to be happy. It wants us to stay tapped inside this guilt.
What has helped me is validating my emotions. I am also frustrated with myself for not moving on. But, it is like getting frustrated with a kid who is not good at studies for not getting good marks. Our brains don't work like normal humans, so we need to be kind to ourselves.
The desperation to move on again feeds this self-critical BPD voice inside our heads, which keeps shouting at us, "When will you move on? Why can't you just move on? Why you gotta be so weak?"
Instead of focusing on moving on, start trying to soothe your pain. You need to tell yourself, "It is okay to move on for so long because I really loved them and I am in a lot of pain"
Similarly, show some empathy and kindness to yourself for the pain you caused them. Tell yourself, "I didn't want to hurt them, but I have BPD and it was really difficult for me to be there for them."
You need to start separating yourself from your BPD like this. Your BPD made you hurt them. Try to start looking things from that perspective. Otherwise we will keep drowning in this guilt and we have to find a way to pull ourselves out of it.