I have been taking 4.5mg for over 3 years and like you (SPMS) believe I would be a lot worse without it.
Have had progressive MS for about 6 years (25 years altogether) and believe that swank/jelinek diet have also helped. However, I still have old damage - weakness/spacitity in right leg causing mobility problems.
Since starting LDN, I have had marked improvement in bladder function, numbness and tingling/buzzing symptoms have disappeared and I have more energy and less fatigue. I also believe that LDN has stopped progression of my MS and my symptoms remain stable.
It is the only treatment I have taken for MS and I will never stop taking it.
It is certainly worth a try, especially for those with progressive disease. LDN has proven to be a safe, non toxic, cheap ,oral alternative to current dmd's.