Your weightloss was NOT the problem with your relationship falling apart. Sure, she may have blamed that for it, but the real fact is that your partner had insecurities and I would go so far as to say, quite likely mental health issues including anxiety (even if undiagnosed) given that she said she had moments where she felt suicidal and was afraid that you were going to leave her.
Another ex's response to your needing to lose weight "Oh no! I'm going to feed you lots of stuff!" - a lot of people make those comments but don't mean them.
I think that you need to focus on YOU and YOUR health/weight issues. I wonder if when you lost the weight the first time round, if you 'threw yourself' into it, then, too. People who make a lot of changes all at once will often fall off the wagon after having reached their goal. It is better to slow things down and let the weight come off slowly because it is far more likely to stay off for the long-term. The reason is because you are actually LEARNING a healthy lifestyle which becomes a life-long habit, and not focusing on just losing weight.
About 7 years ago, I changed my diet and went to the gym. I lost 60 lbs. About 4 months ago, I went to a doctor's appointments and realized I had gained back all the weight I had lost years ago. It was a big shock to me, and I threw myself into action. I realized I was deluding myself that I was being healthy when I wasn't--I'd have a salad here and there, but unhealthy stuff had crept back into my regular diet. It was a painful realization, but I took action. I started going to the gym three times a week, cutting out processed from my diet, and eating less. I've seen results, and I've lost about 40 lbs so far. I'm not quite down to where I was, but I'm getting there.
In that time, my relationship fell apart. I suspect the weight loss had a lot to do with it, because that was when things took a turn for the worse. She confided in me that she had moments where she felt suicidal. She said she was afraid I was going to leave her. She said she was afraid that our divergence in health habits was going to be a deal breaker for me.
We ate out a lot, and on my diet, there weren't as many places I could go. I started picking healthier places, and she started protesting. This never happened before--previously she was never picky about where we ate out at before. I started cooking healthier stuff at home since it was easier to manage calories, and she refused to eat what I made.
It fell apart and we aren't seeing each other a month ago. I realize that my choice to lose weight causes a lot of tension in my relationships because no one I dated before her approved of me losing weight. When I said I needed to lose weight to an ex in the past, she said, "Oh no! I'm going to feed you lots of stuff!"
I just wanted to share. And, I wanted to reach out to hear other stories. Have you ever had a relationship that was negatively impacted by weight loss?