Me and my Master are considering bringing another person into our play, but I'm insanely jealous. He vowed to be mine always, and I trust him. But I don't want anybody else to touch him. How can I get used to this idea?
Okay, back up: It's not a given that anyone can just "get used to" being nonmonogamous. Some people simply aren't wired for polyamory or swinging. The fact that you're having a D/s relationship is immaterial—if you want monogamy, no one can (successfully) order you to feel otherwise.
But, obviously, people in monogamous relationships also experience jealousy, so working to overcome it can benefit anyone. We romanticize jealousy as if it proves love, but saying "I'm jealous" really means "I'm intensely afraid that I'm going to be abandoned by my partner." No one wants that—but for some people, that primal fear is more frequently activated. It can be something as trivial as a look or a smile your partner gives someone else or a platonic friendship. Whatever the trigger is, your animal brain—which equates abandonment with death—responds as if there were a snarling tiger in the room. It shuts down higher mental functions and dumps adrenaline into your system for a fight-or-flight response. Whether you're polyamorous or monogamous, that's not pleasant, and it's certainly not useful in resolving relationship issues peacefully.
I've had flickers of jealousy, but they're always brief and easily soothed. However, I know how it feels to be deeply fearful of something that other people regard neutrally or even pleasurably: I'm afraid of flying. I had a bad experience as a child, and after that, simply being on a smoothly flying plane was an exercise in barely restrained terror. If there was any turbulence whatsoever, I disintegrated into shuddering, uncontrollable tears and nausea. When people said, "Just calm down, there's nothing to be afraid of," it made me so angry that I wanted to hit them. You cannot soothe people with rational discussion when their animal brains are telling them they're about to die.
Now, I could have chosen to avoid travel by plane, but I didn't want that limitation. I tried antianxiety pills, but the medication left me groggy yet still terrified. So I underwent both cognitive therapy and a series of hypnosis sessions to treat my phobia. The combination of all three worked. I'm still somewhat nervous, but not unmanageably so.
Planes still do the same things they've always done; I just changed how my animal brain responds to it. So here is my advice about conquering jealousy: Don't look at it as a problem that's solved by changing other people's behavior. Regard it as a phobia and seek help from people who specialize in treating them. Only when you can view the world of nonmonogamy without that intense, crippling fear can you decide if you want to venture into that territory.