I've been in an LD (off and on) poly relationship for 5 years. We're currently going through our longest haul - 10 hour time difference, 5,000 mile flight - for nearly a year now.
I have had a few experiences where my observations or other feelings were overlooked for the reason that I may or may not be feeling jealous - a feeling I believe to be completely natural, maybe most especially in an LDR situation, when I'm not attending to my relationship as much as I normally would. In those instances, the obvious point was being overlooked. There is a clear separation between an instinctive feeling ("He spends all of his free time with other women and I'm jealous") and an observation ("When he's around this woman, he acts impulsive and always ends up getting hurt").
My boyfriend has gotten involved with two taken women (I was aware of it, but their partners were not.) The last time it happened, I was really frustrated, because it ended badly and I, too, had to cope with the aftermath. But I didn't want to tell him "I don't want to hear it" when he came to me for support - he's my best friend, and I want him to know he can count on me. But it IS painful watching a loved one make a mistake - especially the same mistake twice.
After giving it some thought, this is basically what I decided (and told him):
I'm in his corner no matter what, and I will do my best to control my insecurities and try not to be overly protective. I have my own opinions about what's best for him. They're just my opinions - but they are difficult for me to change. I am hoping he will find someone kind and affectionate, who won't judge him, who will be honest, who will build him up by appreciating his various strengths and interests, and will hopefully have compatible ideas to mine as to looking after his best interests, such as protecting his other friendships and relationships. Someone who helps him succeed in all areas of his life. But I also know that his expectations are different, and I'll do my best to accept and respect whoever it is that he chooses to bring into his life, with the awareness that it's something or someone that makes him happy.
If I have a concern, I remind him of these things - so that he remembers where I'm coming from, and he can understand my rationale and respond accordingly, instead of responding to irrelevant jealousy. But it also reminds me that it isn't my choice who he dates, and as long as I am committed to him, I am committed to his mistakes.
Not sure how helpful that is - this is just my experience - But maybe now you feel like you aren't the only one?