I can definitely get this "off" feeling. I myself, as a mono, feel disconnected from my relationship whenever i know or think my partner is with someone else (i like to give space and privacy to him, so i don't always know where he is). He always returns very happy to me and i enjoy that, but a hint of pain remains there. I think it helps to focus on the good things out of this situation, like your good relationship with him rather than his other relationship, and the nice time you can have when he is feeling happy.
I don't really have a more valid advice as i'm very new to this myself. Talking with other newbie struggling monos seems like a relief thought!