As far as my first major relationship, she stopped being intimate with me not long after we moved in together, but I was so blinded by love that I believed her excuse that she just had a low sex drive, despite the fact that we abruptly stopped having sex after a period of having sex often, and despite the fact that she flirted with nearly everyone that she came across, whether she was in my presence or not. When she left me is was 100% unexpected (she came home from work one day with her father right behind her and started packing her shit, refusing to talk to me about why she was leaving, despite the fact that when she was on her way home I had called and told her what I had made for dinner, asked how much she wanted, and let her know that I had picked up the mail, and she told me how much dinner she wanted and acted as if it was going to be another routine night), but I now acknowledge the fact that if this happened to me now, I would be well aware of what was to come. At the time I was ignorant of where our relationship was headed because she was my first love and I was still deeply, intensely in love with her.
To answer the second part of your question, I don’t think there was anything I could have done to stop our relationship from falling apart. She is a serial monogamist – she gets very involved with every relationship she is in, and even asks them to marry her, but always has left them when she becomes interested in someone else.
With my most recent failed relationship, yes, I know when it started to fail – not long after my daughter was born. She thought she was prepared and excited to become a parent, but she did not heed my warnings that it was not going to be all rainbows and sunshine, and the years of sleepless nights and heavy responsibility was something she was not truly prepared for or willing to commit to. She was consistently immature and selfish, and my gentle reminders of what the true priority (the child) was eventually turned into resentment on both sides of the relationship. It was over long before we actually broke up.
Again, I don’t know that there’s much I could have changed to make it work. She simply was not ready for the responsibility of child-rearing.