Dealing with Relationship Anxiety/Nervousness/Jealousy

I have recently started a relationship with a girl that I am truly in love with; I love everything about her and she makes me incredibly happy. I've known her for about a year and a half, we have been dating about 2 1/2 months (01/24/13). So we are looking at a very young relationship; I'm recently turned 20 and she is 18 (senior high school). Lately, the past few days, I've grown increasingly aware of the fact that I am in a relationship. I don't know if that make sense, but what I mean is, I started becoming more and more aware of the fact that I now have something to lose. I've kept to myself about it and have not let it bother me until now. A little background on me, this is my third exclusive relationship. First was a 16 month relationship that ended horribly, second was even worse. My second relationship lasted 3 months, it's been about a year since the break up. This woman literally effed me up in the head until the point that I no longer believed in relationships until my current girlfriend came along; and, lucky me, I recently found out she cheated (2nd relationship girl). On to more current events, my current girl has this ex that she broke up with a while back, and he keeps talking to her. I'm fine with that, she assured me when we started dating he's nothing to worry about, so I never gave it a second thought. He keeps coming up in conversations now, over the past few days she's told me he has told her that "she is the love of his life" and "he'll never give up on her until she is married." This kinda irritated me, along with the fact that this last conversation she was constantly texting (number 1 pet peeve, I hate it). I wasn't going to say anything until we were about to get off the phone, I told her I wasn't in the best of moods so she asked what's wrong. I told her "This kid is kind of bothering me." Exact words. She assured me and that's that, I shouldn't worry anymore. As of this moment, hour after the conversation, I am feeling really crappy. I'm worried that this relationship isn't going to work, I'm worried about why is this kid constantly texting her; is it her provoking his behavior or if its just him being a douche. I don't want to worry; Anyone have any advice? Could my current mood have something to do with my ex that cheated? I hate this feeling. I don't believe it's jealousy (been there, done that, not worth it) but I believe it's an insecurity thing...Thank you in advance.