Hi and welcome,
IF your brain MRI and neurological assessment were normal then it's highly unlikely to be caused by a neurological condition like MS, MS literally means many scars so there should be lesions found on your brain and or spinal cord MRI's for MS to be even possible. Also in regards to MS, if your brain MRI wasn't suggestive - consistent, even an abnormal LP wouldn't put MS at the top of your potential causes list because there are other medical conditions that will also cause the same-similar LP results. Unless something somewhere in your testing is abnormal to warrant an LP, its probably unnecessary to under go an invasive test like an LP is at this stage.
I actually would of expected your brain MRI and cognitive testing would of shown up signs of FTD for it not to of also been ruled out like Altzimers was, i'm unsure what medical conditions could cause such a rapid decline over approx 6 months if all your test evidence is normal. Honestly, i think your best option with mental health causation being put at the top of the list is to specifically see a psychologist or psychiatrist to get this idea ruled in or out just like you would for any other potential medical explanation...
Keep in mind that there are mental health conditions that can cause these types of symptoms eg conversion disorder, health anxiety etc so it's 'possible' but possible doesn't mean it definitely is, having the proof of a mental health causations being ruled out, not only would it be good for your own knowledge to know for sure that isn't the cause, it will also help your dr consider any other alternative causation that you have yet to be tested for.
I hope that helps.......JJ
btw many diagnosed people with MS have also had psychological assessments done whilst going through their diagnostic testing for MS, so please don't automatically dismiss the potential benefits a psych assessment could provide you.
I went through all of this and after eight years I went to a psychiatrist and found I had suffered a Midlife Autistic Breakdown. It occurs to people who spent their whole life never even knowing they even have Autism Spectrum Disorder. It can be very devastating and can make you catatonic. I know the stigma of being labeled "mentally ill" is scary but knowing is better than not. I am now on medications that allow me to function somewhat normally but I am still to disabled to work but no longer have a death wish. You can go online and look up ASD and Sensory Processing Disorder and compare your symptoms before you seek out a Doctor. I also have Trigeminal Neuralgia which can mimic ocular migraines. Hope this in some way helps