Yes I had something similar towards the end of one long term relationship and there was someone else. It was like my ex had a sudden personality transplant and just viewed everything very differently, and re-filtered trivial scenarios so that I was controlling etc.
Her attitude to lots of things suddenly changed - she was much more flippant and mean about other people in general, her taste in music was different, she was eating red meat which she would never touch before and was drinking more which she never used to. As well as the attitudes she even started adopting the mannerisms and vocalisations of the OW. (I was aware because I knew of the OW in question.)
To my mind she was quite manipulated by the OW, and she was at quite a vulnerable point due to the recent death of her dad. But I am sure her take on it was that the OW was opening her eyes up to what an awful person I was. Well if you're being lovebombed by someone new and exciting I guess the lens can change on routine interactions with a boring long term partner!
It was a very hard time for me to be honest, because I was trying to reconcile my understanding of myself with what she was saying, and I was haunted that I was so unaware that I was somehow this horrible person, despite my ex never having had any issues before or there being any indication that anything was amiss, or that she was anything other than steadily happy in the relationship.
But saying anything just plays into the new narrative in their heads. I remember thinking at the time...I can't even try to defend myself or have a discussion about our relationship because that will just be seen as evidence that I'm controlling!
Best of luck with it OP, I don't think it's a foregone conclusion the same scenario is it at work for you, but it may help to hear different experiences. It was the beginning of the end for my relationship, but I made my peace with it as I couldn't be with someone who was suddenly treating me the way she was, coupled with deception of her emotional/ physical? affair with the OW... I knew I had done nothing to deserve that.