If you aren't happy with the gf - then break up with her. Don't start looking for something else while you haven't cut ties with her yet. You're complaining about her treating you like s*** but you are treating her like a piece of s*** too.

Polyamorous relationships are definitely harder to find and establish than other relationships. I think the fact that you're engaged might even hinder some others - not everyone will be ok with participating in an unequal relationship.

What does your fiance think about your current gf and the fact that you're looking for another one behind her back?