I said no, would you want me going out without you

You fucked up a bit here. You justified your position when you need no justification. Just say no and leave it, she understands why the fuck you don't want her going out. Even if there is no proper reason, if she truly respected you as a man and a leader she wouldn't question your authority. Essentially this is why you get this feeling that she is losing respect for you - because you do not even respect yourself and your own decisions and need to justify such a basic thing that she knows the answer to/

Get rid of that bullshit media brainwashing that women need sexual variety like men and need more dicks. She doesn't need to see what other dick is like if you are providing her with everything she needs - rock solid values and beliefs, confidence and a good rough fucking.

Best course of action now is to get her to start respecting and wanting you more by improving your 'game' - then you have a choice of having a decent girlfriend or having a decent plate.

slutty friend of hers

Dude slutty friends are a red flag. A girl's peers are the biggest influence in her life.

Going out to a club by herself is really bad.

When I see a girl walk into a club by herself, I know she wants men to approach her and possibly take her home. All guys know this and swarm like piranha upon the whiff of blood in the water.

I hope she was just testing you and she wouldn't get dolled up, dress in a short skirt, and then proceed to walk into a hook up spot by herself with you at home and then drink and talk/dance with other men.

If she does this, I'd immediately next or demote.

Nobody goes to a club, without her boyfriend, for the ambience and overpriced drinks.

When a girl goes to a club (especially alone, but also if with a slutty friend), she is trolling for male attention and free drinks. Even if she doesn't think she's intending to cheat on you, she's very definitely intending to meet and talk with guys, enjoy the attention, and mooch some drinks. Everyone knows guys hit on girls at clubs -- especially girls who are there without their men.

She may not think she intends to cheat, but sometimes, a girl can meet a guy who changes her mind.

Keep it simple:

"My girlfriend doesn't go out to meat markets without me."

"B..b..b..but don't you trust me? Quit being insecure!"

"I'm not ordering you around. You're a human being. You can do whatever you want. But my girlfriend doesn't go to meat markets without me."

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Establish boundaries with her in a calm and stern manner, if you have been applying rp principles long enough(LIFTING, frame, smv boosting lifestyle etc) it shouldn't be an issue. If she isn't going to comply then demote or move on, if you have abundance you will be fine.

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"I'm not attached to her as she is to me."

Really? She seems to be trolling and looking for ways to find strange dick while you are posting rants on the internet because your bitch refuses to obey and doesn't want to stay at home with you instead of a night out on the town.

She is doing what you, as a TRPer, is supposed to be doing establishing abundance and imposing dread on her SO.

Text her that she's a fuck buddy now and can do whatever she wants.

Judging from her actions, she doesn't look like LTR material as of yet - she is 18 after all.

Not LTR material means you should be plating her, let her do w/e she wants and you should be spinning other plates. If she likes you enough she shouldn't be dreading you by going to clubs alone and shit.