Thanks for sharing your experiences. I don't know if I can be any help, and I *know* I can't offer any solid advice, but tonight my GF is out with her lover also. Normally I'm home and our rule is that she must come home from play time to sleep in our bed together---but tonight I'm visiting my parents so I won't get to see her.
I find it very hard to fight the feeling that I'm being left behind, or taken advantage of. I love my GF very much, and I don't desire anyone else. Perhaps what's so hard about the whole thing is that I sometimes find it hard to understand why she would want to be with someone other than me if she loved me like I love her. That's sort of a cyclical negative thought process though, so I remind myself of how good it feels when she's around, and that she really must love me.
It is very good that you're exercising and being healthy. That's important.
I am not yet sure if I'm truly mono or if there is some poly lurking inside me somewhere, waiting to emerge for the right person. I have a lot of inhibitions in my interactions with other people, and I find it hard to overcome that to even begin to think of people as potential romantic interests.
It sounds to me like you are exploring polyamory for the sake of your husband (or your marriage). That is admirable that you're willing to go the extra distance for someone you truly love! Try not to let it fester and become a source for resentment. I am not sure how to best accomplish that---I'm having a lot of trouble with that myself at the moment.