Having feelings doesn't mean you're not cut out for poly.
Everyone has feelings. Often the 'most' poly people, definitely the most experienced poly people still get jealous feelings or upset feelings. We are human beings, we have feelings.
It's what you think you need to do about them, and/or what you actually do about them, that might affect your poly-ness. Sounds to me like you are working at expressing your feelings, and he's making safe space for you to do that with him. That's a good thing.
There's lots to read on these forums about jealousy, and other feelings, and things to do (look inside, talk them out, distract yourself, get therapy, and probably a hundred others) with feelings. I recommend you read, read, read!