He keeps flirting with my girlfriend...

Nah bro, you SHOULD be worried, in fact, be very worried.
My advice? Get out of there ASAP, because if shes doing things like this, AND you told her how you feel about it, and she continues the behavior its not looking good for you my dude.

Shes either gonna eventually cheat on you, or just outright leave you for another guy, not necessarily your friend here either, but IMO she seems like shes already trying to fill your void.

First off if this guy is flirting with your girl right in front of your face, that's a huge disrespect and you as a man need to confront him about it. Be like "dude why are you flirting with my girl?" if it escalates into an argument make sure you land a nice punch on the nose and you're good to go. That douche just can't be goin around flirting with his "friends'" girlfriends.

Speaking of girlfriends, you need to confront yours about why she's flirting with this guy back and why she keeps bringing him up when you already made it clear that it bothers you. If she acts like a little innocent idiot, then tell her she needs to learn a little about respect for her significant other and dump her right there.

It sounds hard to do but you just have to do it. Right now they are both walking all over you because you're introverted and would prefer to stay away from uncomfortable situations. But everyone's gotta come out of their safe zone once in a while, and this is the time where you need to do it.

Do me a favor, get on your knees, and thank God right now for giving you a sign that is so bright and clear. Seriously, you're so lucky that she's giving you red flags in your face rather than behind your back. Yea, it's pretty disrespectful for her to throw hints in your face like that, but like I always say, better in your face than behind your back.

Anyways, my advice is, you should CONSIDER this relationship over but you should "ride it out" to see how it goes. Personally, I think if you have a gut feeling that your relationship is over, just keep it going and gather all of the red flags and evidence you can. Ultimately, that being said, it will only make your gut feeling that much stronger in a future relationship. For example, if you leave her now, you'll never find out any additional hints she may have dropped prior to potentially cheating. On the other hand, if you keep things going, and for example her behavior becomes even more strange and ultimately decides to break up with you or cheat on you, then next time you look for a mate you'll be smart enough to look for these red flags before you begin dating.

Get as much information out of her and be as cunning as you can. As far as your guy friend goes, before you befriend his ass, since this relationship has basically turn to garbage, consider him a former friend. I would allow them to flirt and gather intel. Since you're girlfriend is stupid enough to tell you what she likes about him and what he does, she's doing nothing more than ratting him out. Punch him in the face for flirting with your girlfriend (not literally). A real friend wouldn't do that.

It disturbs me that you think about this everyday. Seriously dude, I don't even know you and I'm concerned. That's disturbing. Don't you ever let a girl ride your feelings like this again. Real women know their boundaries and would never be so flirtatious with other guys. This is seriously borderline cheating.

Lastly, @VioletTrees has a good point. Since you're so compatible with your douchebag friend's gf, you guys should switch. I wouldn't remain friends with him afterwards, however.

You should trade significant others with him.