Brief Answer:
Possible stroke, more info on test results are needed.

Detailed Answer:
I carefully went through your query and I am sorry about the problems you are still experiencing.
While a definite diagnosis is difficult to assess without knowing the tests you have done, TIA doesn't seem compatible with your history. TIA is an acronym for transient ischemic attack, meaning that the symptoms are transitory and have fully disappeared (usually in a hour) and this certainly doesn't seem to be your case as the symptoms persist. You seem to have a left side hemiparesis (weakness of your left limbs), while these fits you describe could be a case of a partial motor seizure.
Judging by what you describe and if these symptoms manifested themselves abruptly, then the most probable diagnosis is a stroke, which could be either ischemic (interrupted blood flow in a blocked vessel) or hemorrhage (rupture of a vessel in the brain with leakage of blood). This is precised by brain imaging, but both are serious conditions which have to be addressed as to their probable cause, further exams and treatment.
However for any further discussion it is absolutely necessary to know the results of the brain imaging, be it brain CT or even better brain MRI. They are needed to distinguish ischemia from hemorrhage (signs and symptoms could be the same) as well as to exclude other possible diagnosis (especially important since stroke is not common at your age). You say that the tests are negative but you say nothing about which tests.
So please go back to the documentation for more info on brain imaging results, if they have not been performed they should be as soon as possible. Also check the documentation about possible EKG, heart ultrasound, ultrasound of neck vessels, blood tests etc because they are necessary to discuss possible causes and further necessary tests.
Other info on your medical history could be helpful as well such as other medical conditions, smoking, use of drugs, contraceptives, recent trauma, migraine.
I remain at your disposal to discuss further management after you provide more info. Looking forward to hear from you again.

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Brief Answer:
Detailed Answer:
Hello again. Thank you for providing more details of the tests results. While clinical history is important, exams are also a crucial part which could change the whole picture like in fact in this case.
From the history I had assumed it was most likely an ischemic stroke which in some cases the commonly employed CT might miss, but I see now that you have undergone extensive brain imaging with MRI, with MRA evaluation of the neck vessels as well, with normal results. So now I understand better the decision of your doctor not to start any treatment for the moment.
The good news is that you do not have a stroke. The MRI would usually indicate also other serious conditions like a brain tumor or multiple sclerosis. On the other hand it's not a case of an easy straightforward diagnosis I must say.
The most probable explanation of the shaking/flailing of your limbs might be a partial epileptic seizure which can at times be followed by transitory weakness of those limbs. However it must be confirmed by EEG, so the next diagnostic step in my opinion would be an electroencephalogram (EEG).
Also, regarding the MRI, I am noticing that (apart from the one of the neck) you say without contrast. That is unusual because for the MRA of the head usually contrast is employed (although there are at times used non contrast techniques). So, if indeed no contrast exam has been performed for the head I think that should be done, because contrast might evidence previously missed involvement of the brain or it’s covering membranes (meninges) by a tumor or an infection at the origin of the seizures. Those being missed by MRI w/o contrast are improbable though, so I don't want you to panic and worry too much, just considering all the options.
For the moment until at least the EEG I don't recommend starting a treatment. Since you mentioned your psychiatrist I am assuming the benzodiazepines found in your blood and urine have been prescribed by him so he has advised you on their further use.
I hope I have been clear in my explanation. Don't hesitate to come back for further questions. I wish you good health.

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