We use "I just assume not" when we've had a bad experience already and aren't keen on repeating it. It expresses a frustration to mild annoyance on the part of the speaker, as if saying "if you don't mind" or "if it's all the same to you" - "i'd rather not..." I don't know if that's in the Russian translations, but that's the attitude of the speaker.

@Olivia that was my original idea, but I wasn't sure and went with a more literal translation. Better one would be "я бы предпочёл это не обсуждать" or "мне не хотелось бы обсуждать дальше".

Olivia, thank you for the clarification - that's precisely the part of the sentence I was interested in.

Zero: Thank you for those variants - they sound much more emotional and accurate :)

It's not that you don't see the point - it's more like you don't want to get back into it for obvious reasons.