Frozen shoulder at 18?

Where are your parents in this discussion?  And you are right to be educating yourself about your health and your conditions.  If you had difficulty moving your shoulder from pain/injury, and you protected it by not using it (immobilized), it is possible you could have developed a "frozen" shoulder in addition to any damage sustained during initial injury.  Frozen shoulder is generally a result of scar tissue formation, which usually results from healing of a previous injury.  It can be from a major acute injury or from repetitive stress injury (overuse).  Check this out: https://radiopaedia.org/articles/adhesive-capsulitis-of-the-shoulder.  So there are signs on an MRI that can indicate a frozen shoulder.  These could be questions you could ask your doctor to explain to you.  Unfortunately, with today's healthcare's state of affairs, she may not be willing to sit down and explain anything given their limited (self-imposed mind you) window of about 12-20 minutes per patient consultation and being flooded with a glut of baby boomers.

To be honest, I'm more concerned about your initial injury and your labrum.  Sounds like a previous dislocation if she sees "a line" or fissure.  This is something to confirm with her.  And hopefully, any rest you gave it after injury allowed the injury to heal to a reasonable state.  And still hopefully, all that you need to do is reform the scar tissue with physical therapy to reduce pain/swelling and eliminate that "pop" you are experiencing, which sounds like impingement.  Again, something to confirm with her.  Physical therapy is usually the conservative route to go before trying anything interventive like surgery.  They usually only recommend surgery if they see something readily apparent that they can fix like a tear or something that they know physical therapy wouldn't help.  And even then, immobilization can still work as good as surgery if done properly and you are still in the acute phase.  Frozen shoulders usually aren't considered acute.  Generally, you would want to seek a second opinion if her prescribed treatment doesn't help, made things worse, or she admits she doesn't know what is wrong nor what to do.  And in that event, gather as much information as you can (medical records) she has maintained to transfer over to your next doctor.  Then your next big challenge would be in deciding who to pick for a second opinion.  And sadly, all doctors are NOT created equal, even if they do take the same boards and hold the same licenses.  They all share the same basic level of competence because of those regulations, but there is still variance in some being better than others.  Factors such as gender, race, nationality, age, current patient load, or just general background can play a role in just how effective they are.  And again, at your age, I HIGHLY recommend you seek the help of your parents or a trusted relative.  You may be of age, but I seriously doubt you fully understand what is going on.  Most patients, regardless of age, most likely don't understand what is going on.  That's why it helps to have someone accompany you who will try and understand the problem as well.  That requires them to be actively engaged and actually putting themselves in your shoes.  Not sitting in the waiting room twiddling their thumbs.

I was in your shoes once upon a time living away from your old home at college with an injury.  You are NOT alone in feeling scared to ask questions.  It happens to me too.  It drives me nuts.  For one, you don't want to risk irritating them, you want to feel they are on your side.  Two, you might not like the answers they have.  But...these are not excuses you can accept if you want to make an informed decision about your health.  And that's one reason why you hear the phrase; "give it to me straight, doc".  Meaning tell me what the problem is, don't sugar coat it, and make sure I understand it.  So what I've done, is prepared a list of questions that I pondered prior to a consultation and will try to go through them one by one and even take notes to be sure I get an answer for each one  Be careful with this list, if it's too long, they may not allow you the time to go through them all.  In that event, sometimes they'll assign you to a nurse or physician's aide to sit down with you to answer them to the best of their ability (although sometimes these questions can ONLY be answered by the doctor in which the nurse/aide will followup with you after consulting with their doctor for select answers).  Sometimes they have resources available for patients to research themselves (although sometimes these resources will tick me off because sometimes they only contain general information that any idiot could find with a general Google search, I doubt some of them in the office have read the resources themselves).  It might be a good idea to ask them beforehand if they'll be able to answer your list of questions and to see what alternatives they offer if an appointment simply isn't enough time to go through them all ie. leaving the doctor a copy of the list and having them mail you back a letter/email with their responses.  That's also why I feel it helps to have an advocate(parent, relative, friend) present to help recall anything you might have missed or forgotten.  It is also perfectly legal in most states, except for California and Florida (this would be something to verify in the state you reside), to record audio of your visit/consultation without your doctor's prior knowledge or consent.  They most likely won't mind anyway if you DID ask and explained your reasoning why (bear in mind if they refuse, you will have to decide whether to continue with the consultation or not without the recording).  This could be as simple as hitting record on your phone, setting it aside, and letting the conversation continue as normal.  Whether this recording can be used in a malpractice suit, I do not know.  But I assume getting a healthy shoulder is more important to you than getting a settlement.

Doctors here in the US are on the whole good people.  You would be hard pressed to find one that didn't care about their patients.  So it is a safe assumption that they want to help you get better.  That does NOT mean that they CAN help, that depends on their level of expertise and the complexity of your problem.  This is something you can begin to verify with independent research or by asking them directly.  Like "are there any other treatments that you know of?"  This also further stresses the importance of having someone there with you, in my opinion.