Thanks so much for your very thoughtful replies, idealist & london - I am so appreciative!
I modified the original post; I don't actually disappear for weeks, that was just Boyfriend's words/feelings. In the last intense relationship, I simply wanted to see the new guy every 1-3 days, spend the night, etc., and that was a bit too much for Boyfriend, who prefers me to see someone once every 1-2 weeks.
Great point on "time vs intensity". I wouldn't say that I lost desire for my SO when seeing someone else, but perhaps I was paying him less attention than usual (esp. seeing as we are usually together almost 'all the time'.) This time around, I want to try harder to please my SO & make him feel loved & wanted.
Honestly, sometimes I think I would do better being a single poly person. It's just hard to even think about that when my SO says he will never speak to me again if I move out of the house we share. He's not willing to see me if I become a single poly. Sigh.