Very well said, Airen. To add...although not in a Poly relationship, we have been in an open relationship and I have had my share of "concerns" and jealousy since we started a couple of months ago. These have been rooted in myself only and she has done nothing to cause them...in fact she has done the opposite - tried to keep me informed and given me the option to tell her "no, I can't handle you seeing him/her right now". I think the issue for me is that this is so different than the long life of monogamy I tried to live (unsuccessfully), the realization that I need an open relationship, her ability to find partners much easier than I can, and my lower self-esteem telling me that I am not good enough fer her. I know this is my internal critic seeding these doubts in my brain.
We have had many discussions about this and with our recent 3 week separation due to a project she had to work on out of state, I have really learned how she feels, why she feels, and how compatible and good we truly are together. In essence, that internal critic has been silenced. That said, I felt a need to have her mostly to myself the first couple of weeks she returns and she has agreed to my request and said she fully understands. She even offered to be with me more than I requested, but I do not want to bogart her time completely.
I have learned to take any fear I may have and examine the cause of it. Take that cause and either eliminate it or find a way to reduce the impact, which then reduces or eliminates the fear and jealousy. Open relationships require open and honest communication, otherwise they will be fraught with fear. Our communication has always been open and honest with each other, I just need to do the same more often with myself now!