Oops, I'm using my girlfriends account, my username is actually polypenguin
Anywho, Shooting Star, I'm 23, and know where you're coming from. Your question which is by far the most common question for folks new to polyamory or even the idea of polyamory is this "How do i deal with the jealousy?"
This is the most difficult question you will have in learning to be poly, or be with someone who is poly. But the question is inherently WRONG, the question you need to ask is not "how do i deal with it," but "WHY does this make me jealous?"
In other words, think of a situation where your girlie is doing actions that result in you feeling jealous (remember no one can MAKE you feel anything, you are the only one who can EVER make you feel anything). Now, let's say for example you thought of her having sex with another person. What is it about this action that results in you feeling jealous? USUALLY, the reason comes down to insecurity, that scared feeling saying: "What if he's better than me? or What if he has a bigger dick? or What if she feels more of a connection with him or her?" and so on. You need to find out why this would make you feel insecure.
Remember, sex is sex and love is love, some people like to have it with one person, some people like to have it with multiple people, some people like to have it with animals. The point is, polyamory is not about one person being inadequate, it's about realizing love is NOT a zero sum game. Because you love one person, it absolutely does NOT mean you cannot love another.
For example: I had someone tell me once when we'd gotten onto the topic of polyamory, i could not love more than one person, i responded "Do you have any children?"
"Yes," she said.
"Three," she responded.
"Which one do you love?"
She knew what the answer was of course, but knew also the ramifications as well.
You are wonderful, and amazing, and ANYONE would be lucky to have you.
If you would really like to learn more about polyamory, the best book by far I've found is "Polyamory: Roadmaps for the Clueless and Hopeful." It is the most real take on the problems/benefits of being polyamorous.