Don't worry! Poly isn't (too me ) all about notches on a bedpost. In fact, I haven't met many (if any) poly people who are sluts. Most people on here don't seem so. My husband's GF only sleeps with him (and he with her). My two gay friends always play together, and the poet I know has two husbands (no other lovers).
To me, the primary issue sounds like your security with your husband. What is the thing in that relationship that makes it solid? How is your relationship with him different from others?
I don't get jealous (too much) of my husband's GF because I know that we have something singular that he can't/doesn't have with other people. I DO get more possessive of lovers when the relationships are not yet stable. What cures jealousy for me is acknowledging deeply that my relationship with anyone cannot be replicated. Does that make sense?