Guys are constantly hitting on my GF.

So I am a stocky guy, (23) and my GF (21) of 3 years is very attractive. Some people have mentioned that we seem mismatched, that they pictured her with a more Abercrombie type. But nevertheless, she loves me and I her. Anyway, a situation always arises whenever we go out, or when she goes out with friends. If I leave her to get a drink, bathroom, etc. a guy will inevitably come up and hit on her. Its like they don't believe she's serious about being with me. Its worse when she's with her friends. She normally politely turns them down, but she is still naive about what random guys at a bar are looking for, so she'll tell me stories about how she chatted with a guy because they work in the same building, have a mutual friend, etc. and the guy always tries to get her number or invites her to the next bar. She's just starting to experience the bar scene, and I'm already nervous/jealous because all of her friends are fairly promiscuous and although I trust her, I am nervous for what guys might try to do. On top of that, our perceived "mismatch" is always stuck in the back of my head, and I'm sensing she's becoming frustrated that I haven't made time to hit the gym as well as figure out my future plans concerning school and my living situation, how I never plan things, she's currently pretty upset with me over this. We don't fight often, but she's said that the stress I have from work, on top of what I mentioned above is affecting our relationship. When I said I'm going to try harder to rectify all of these things, she says that its not going to automatically bring us back to normal. So, two part question: How do I react when either she tells me a guy hit on her? Do I show her I'm jealous? What is the classiest way to let a guy know that I'm with her without coming off as a dick? Second part: She's upset with me now, I am working to fix all of my current shortcomings, but as far as repairing the relationship, is time the only way to heal this one? She mentioned that we have a lot to figure out, she doesn't want to break up, but she says we need to figure these issues out now or it will never work when we both start our careers.

TL;DR Hot GF gets too much attention from other guys, which makes me nervous because our relationship is struggling