I've read this post 1000x before, but a few "tough brotherly love" thoughts for OP:
-OP's gf breaks the age limit formula for serious dating: your age/2+7. If OP is 28, this would make his bottom limit 21. It's an estimate based on EQ. Not bible, but a good guide.
-The girl is 18. 1-8. Hardly a mature adult and not even close to being emotionally developed, like he's expecting her to be. Yeah, she's going to be all over the place and a high cheating risk, despite any "boundaries" he sets. He needs to accept and understand this if he wants to date her.
-CAUTION! - Controversial SS Statement: She is ALLOWED to have friends, male or female (I hear the SS gasps now, haha). You don't own her. And in this day and age, the line between both sexes is blurred. Plus, these are people she's known since high school so what's the big f*ucking deal? A confident man wouldn't give a f*uck because a confident man has an abundance mentality and knows he can easily replace the girl if she f*ucks up. A confident man would say, "Cool, go have fun with the high school posse. I'm doing XYZ, we'll sync up later. Ciao." And that's it.
- If my gf wanted to swing by when I'm hanging out with my friends, I may also say no. I don't want these two worlds to intermingle on a regular basis. There was actually a famous Seinfeld episode about this.
-Get your own life with hobbies, interests, maybe another business. Get busy and stay busy so you're not so available to just invite yourself wherever she's at to "drop by." That's really weak, dude. Wreaks of suspicion and insecurity. At 28, don't you have better things to do than hangout with 18 year-olds?
-See this girl as a (very temporary) side dish on your huge plate of food. Tasty, yes, but not the main course.
Ironically, the more unavailable you make yourself (within reason) and busy you keep yourself and hang out with YOUR friends, the more she'll respect, admire, and be attracted to you.
Good luck.
~Dash~