Past relationship length as an indicator of future success
Past relationship length would be a very minor red (mayber reddish? pink?) flag for me if the person a) has a past of serial monogamy of relationships lasting no more than approx. a year at a time b) has always entered into a new relationship right after the previous one ended and c) blames the end of their previous relationships exclusively on their partners. That might be indicative of NRE junkiness, imho. But this is a minor trigger for me because of past issues.
As to the OP's situation - it reads as a 'OMG, I've fallen for someone else while in a relationship, ERGO I have to have fallen out of love with my current partner'-freakout. If he's not willing to talk, maybe but it on the back-burner for a while and ask him to self-educate on poly and decide if it's something he might consider?
Me: bi female in my twenties