I have been in both situations, But in all my long term relationships, they were awesome much of the time, contributed much to my life in experience, and provided the contrast to clarify what new desires I have for the future. I regret none of them and appreciate they were willing partners in the dance we had.
I did notice when I was the one thinking it was coming to a close it was still rough but a bit easier as I was thinking about it longer but not wanting to put it in the space if it was just me or my mood and would pass. When it was the other person, it was much more jolting and took more pondering. Since the other person pondered longer and had come to the decision, this makes sense to me. I don't think which way it happens makes much difference. We have to just move forward and take responsibility for what we are doing with the next moment. It is always a dance two people do together mutually and create mutually, the good bad and ugly. No blame or resentment is needed or assists moving forward.

Yes. At least 3 times. Once I didn’t know how close I was to losing her, twice they didn’t know how close they were to losing me until I left.

It is all too easy to take another for granted. That has happened to me with all my relationships. However, after losing my last partner I vowed to never take another for granted again. We all must learn to do things that foster what we have and, counterintuitive as it may sound, this includes having alone time - for both parties.

Don't you think we keep trying until we die? Well I guess only if one truly wants a relationship. Not too many things in life are easy.