Well you are certainly getting some help. I'm really pleased. There's some great advice here for you and IT DOES GET BETTER!!
I came here in May I think, when I was still having bouts of pain just like you have described. I can't believe how far I've come in just two months. I've gone from being really unsure if I even wanted to continue with my relationship to now seeing the positives in my partner being poly (and I'm mono). I was taking a long drive this afternoon and I felt more at peace and more loving and loved than I have in a very long time. I'm telling you this because I believe somewhere in your very long post you wondered what was in it for you. I asked that very same question.
I think the pain you're feeling in the weekends when he's away is probably a mixture of both jealousy and loneliness. I too have filled up my time with things that I would normally enjoy and still felt intense sadness, grief etc. For me I think it's because Z and I are so close and do so much together that when he's not with me I'm acutely aware of where he is.
I've learned to negotiate and that's helped a lot. He now goes as slowly as he possibly can and I go slower than I think I need to. Previously I tried to give him as much freedom as I could and then when he took it all this pain would come up for me.
I'm sure we'll talk more, it's an ongoing process. Just take one baby step at a time and let your partner love you as much as I'm sure he wants to.