Congrats! You are confirmed for being a sentient being. It happens.
I've had a partner now romantically for a few months - He's had a girlfriend for a year. From her behavior, I assume she's very jealous. (Can't really have a conversation with her to confirm or deny. She's turned down a number of free lunches, though we hung out in the past.)
For me, I know jealousy occurs when I see my relationship with that person is stagnating while her/his relationship with another is growing. Am I not worth the effort of being romantic? What's s/he got that I haven't got?
In many ways it's easy for me to date a poly person who's in another committed relationship, because I feel more control over defining the relationship without hurting the feelings of someone I love. My partner was disappointed, but didn't take it the wrong way, when I said I wasn't interested in living with him. He's not putting all of his emotional energy in me, and he has a great girlfriend. Being able to see the positives with your husband having a girlfriend may help, and getting to know the girlfriend may hugely alleviate some of your stress.
I feel very good when I see partners being nice and affectionate to other partners, especially if they've been together years. It shows me the person's capable of respect and love, and isn't just "putting on a show" with me. With mono people, you sort of have to take them at their word they're good partners.
It sounds like in some ways you think of your husband in a very different light, based on your signature - You say you're married, but you have two lovers. Do you maybe expect more security from your husband? Do you expect him to view love and sex differently than "other guys?"
I can honestly say I wouldn't be thrilled if my partner got another partner, even though I just went on a date yesterday, and I'm glad for his relationship with his girlfriend. We're both young professionals, both of us work more than one job, and we only see each other about two or three times a month, on a good month. Having another person take away from me would hurt my feelings. I completely understand from that perspective.