I think there can be terrible relationships in all setups - and that polyamorous relationships are no exception.
I think when done well there is the possibility for it to be wonderful, but it does require a lot of self-reflection, honesty and open communication. So in that it's not for everyone.
I think one of the most common mistakes is to try and prescribe the limits of a given relationship – and doesn't allow for the fact that relationships and feelings often won't happily remain within pre-defined limits.
So, in beginning this, everyone has to be open to changing dynamics, and the possibility that the shape of things will change over time. I think this is true in all relationships, actually, but naturally moreso when there are more than two people involved.
I think it doesn't work particularly well if anyone in the relationship is co-dependent - everyone needs to be pretty independently minded and happy in their own company. It works best as an understanding between individuals who see themselves as such.
I think it's this aspect of it that suits me - I've never been comfortable with the idea of being someone's 'other half'. I'm not looking for someone to 'complete me' - it's my job to complete myself if I find myself lacking.
So I'd say be careful in your choice of partners. Make sure they're being honest with you - but even moreso with themselves. Problems often happen when people say they want one thing but deep down want something very different. Make sure that you can all talk to each other openly and honestly.
And get a functional and robust system for scheduling and co-ordinating diaries!
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