i dont think this is something that you can generalize, in order to understand whats going on. the facts and the answers depend on your relationship, how it broke down, how it ended.
At the end I was ready to live together and rearrange things (but it would not be immediate, I would need a couple more months), but it was too late and they had enough, or didn't believe me, or both. I can't blame them.
it looks like this was a significant source of tension in your relationship. it was in mine, too. we both talked about getting married, but i was never prepared to take that step, or even invest in it, because of how unstable our relationship was. i also withdrew emotionally and physically from the relationship. when she broke up with me, and i made the case for our relationship, i could tell it was hard for her, but she had committed to it.
when a woman (bpd or not) decides that a relationship is in a dead end, and she has partly or fully grieved it, it is hard to come back from. grand gestures can not only feel like too little too late, but even a source of resentment, eg "why couldnt he do this before? why wasnt i good enough?".
i understand that wasnt the only source of conflict in your relationship, or the sole thing that broke it down. im explaining it, in part, from her perspective. its important to see this through her eyes.
the fact that this was an online relationship is a part of that. online relationships, or long distance relationships, or both, can offer the illusion of intimacy, as well as mitigate some of the hard stuff.
were the two of you ever together in person?
During the final months there were also new personal stressors (unrelated to me) going on in their life, to do with their health and getting older, that were sources of stress of them.
this may have been a significant factor too. stress is hard on a relationship, it sometimes forces people apart, and people with bpd in particular dont handle it well. no ones "fault", necessarily, but a hard reality of life and relationships.
They now have me blocked everywhere. I kept emailing and finally they responded, but they seemed like an angry, different, distant person, not remembering anything good at all, and criticizing me in ways they had never done in the entity of our relationship, even during the worst moments.
if a person has blocked you, they are putting up high walls. if you try to get over those walls, they will either build higher walls, or lash out. she may have carried resentment during the relationship that spilled out.
It makes me wonder if I should try to contact them more. Instead I have been depressed and letting several weeks pass, and only tried to email and call them about 10 times.
no, you should not contact her more. it is pushing her further away. give her space, and a lot of it. giving her space, by the way, doesnt guarantee that she will become more open to contact.
from a "trying to reverse a breakup" standpoint, youre in a tough situation, and i know that it hurts, and i know that you want to do everything you can do. but in order to have a chance at that, there is a lot of ice that needs to thaw.
it will help us to know:
1. who did the breaking up? what was the reason given?
2. how long ago was it?
3. what was the last message you sent to her?