Why can't I trust my girlfriend?

Hey everyone.. So I have a ridiculous problem, and I realize that even as I'm typing this, but that doesn't make it go away.. So I'm with an absolutely GORGEOUS girl right now, and I've never felt more in love with anyone I've ever been with. She's a diver, and she has an amazing swimmers body, and she is extremely sexy. The only problem is, for some reason I cannot seem to trust her.. She has NEVER given me a reason not to; and, to be honest, she is one of the most faithful people I've ever met. She ALWAYS tells me how much she loves me and how she will never cheat, but I still have a problem.. What it comes down to is that the way she dresses when I'm not around makes my jealousy just explode.. She wears shorts that are so small that you can literally see the very bottom of her butt hanging out of them.. And she wears them ALL the time.. And the thought of other guys ALWAYS being able to see her butt just drives me insane. All these thoughts begin racing around in my head like "what if some super attractive guy flirts with her because of how she's dressed and she falls for him??" or "what if the reason she wears those shorts is because the attention I'M giving her just isn't good enough??"... Please help me with this... I don't want to tell her she has to dress differently, because that seems very controlling and I don't want her to think she has to change who she is for me.. It's just that from the jealousy I feel about this, it stems into other things like "what if she's flirting with someone over texting right now?" or "what if that guy she was talking to yesterday at diving is starting to like her and she feels the same way?".. Even when she looks at a guy now I immediately assume she doesn't like me anymore and loves him instead... Please help me!!! I know I'm ridiculous but I just cat seem to figure out why in my head..