I've been working on possession myself; in past relationships I have hated feeling like a possession. I had a bf once who used to accuse me of sleeping with the neighbors when he would come home and I wasn't there. I also had another bf who used to freak out if I was gone at my friends house for what he thought was "too" long.
It's tough because even though I have been on the receiving end of possessive people, I can't help but have some of those tendencies myself. I have control issues and sometimes I catch myself wanting to dictate what my partner can and can't do simply because I feel left out or something.
It's important to see him for who [I]he[I] is, not some fantasy, idealistic possession. When you can see a person as the individual they are, the WHOLE person, it is easier to allow them, no - GRACE them with freedom. I can preach this all I want, putting it into practice is a little more difficult because I tend to fall back into my conditioned state of jealousy.
There is a book called "Facing Love Addiction" and if you can get over the Narcotics Anonymous type of talk in it, it has some fascinating ideas on what it means to be in a healthy relationship and steps you can take to know your true self and value yourself, indeed LOVE yourself so that you continue or re-enter a relationship in a healthy way - or decide if you should even be in the relationship.
I find it helpful because I need to know if I can continue in my own relationship with my poly bf, or leave it behind.