Welcome to the group. Everyone here is very supportive and helpful.
Like you I am used to being alone and am attracted to very very few people, so polyamory sometimes sounds like an unusual ideal. But regardless of that, I have never felt that monogamy was right for me. Deep down I never understood or agreed with the concept for myself, even as a child.
I am sorry that your partner was not comfortable with the idea, but it is nice that you two could maintain a friendship.
By the way, love Lacuna Coil.