I once dated a girl with whom I would have wound up in the same situation had I married her. Interestingly, our mutual friends told me at the time that I should leave her, but I thought that once you made a decision to date, that you followed through until it had no chance to truly recover.
After two years of three day cycles from her being elated, to being indifferent, to crying and blaming me for things she did (then apologizing), I couldn't take it any more.
I only casually did things with women over the next 10 years, having been taken through the wringer.
I then met my wife and she was a truly solid person in all good ways. I'm glad each day to be married to her.
Too many have married people that are not stable or who do not have good hearts. I learned from couples I knew to hold off until the right one was found. Life often seemed pretty lonely, but I'm grudgingly glad the many years it took allowed me to be with her.
If she died, I would only marry a woman of good character and heart, like her.
Live and learn. Thanks for your empathy. Older and wiser, and listen to my friends more. It happens.