Your suggestion in the second paragraph sounds good. Most of our fights were more or less caused by extrapolating information incorrectly due to our high strung emotions. Things like she'd mention a famous person in passing, and I would happen to know said famous person had a horrific controversy, and I'd suddenly go on high alert thinking she supported said terrible things, which would snowball into "If she supports that terrible thing, she must support all these other terrible things as well," at which point I'd explode, she having no idea why because she didn't know about said controversy and didn't even like said famous person very much in the first place.
When she'd blow up at me, it was usually because I ignored a load of warning signs that she was in a really bad mood, because I felt bad not helping when she felt bad, but the only help she actually needed was for me to leave her alone until she could settle down.
As for the new guy, the situation is a little complicated by the fact that he did actually try to do the polyamory thing at first, and a bit into that is when he first confessed that he was in love with me. However, we realized that what he was really hoping for was that I'd fall for him so hard I'd decide I wanted to be exclusively with him. Once he realized that, that's when we realized polyamory wasn't going to work and I had to make a decision.
It still disturbs me a bit that that happened so fast, but I don't feel like I should be judging since I was exactly like that with my ex before we started dating. Just fell so deeply in love that I couldn't bear not telling her.
At the same time though, maybe that should make me more worried, because I was absolutely off my rocker back then and the first year of our relationship was almost entirely her reigning me in and teaching me how to approach relationships in a more healthy way.