Hey moominmama, welcome aboard.
Have you consulted a physio or any of the support groups about safe activities to try with your MS?
I know my mum was really despondent after her RA dx, and heard 'don't exercise' from the doc, but what he actually said was far more nuanced - obviously don't exercise during a flare-up or the worst of times, but there may be periods where gentle exercise would be beneficial, and may actually help to prevent furtheration of symptoms.
www.nationalmssociety.org/Living-Well-With-MS/Health-Wellness/Exercise
Obviously you know your own body best, but I wondered if you were in touch with anyone else that was experiencing the same thing.
Anyway apart from the exercise thing, I think you have to hit the 'zone'. You have to be in the right place mentally yourself to actively decide this is something you are going to do, and then stay determined and not let wobbles of the biscuit packet variety put you off - just climb back on the wagon and focus on the end goal. No self hatred for a backwards step, but no giving up.
I might also do a bit of soul searching and see if you have something you can use as a bit of a confidence-boost - what do you love to do? What positivity could help break the negative cycle? You are worth more than bouts of self-loathing - what makes you feel good?
I'm not a SW/ WW fan - and am certainly not a role model in weight loss terms - but I am just (just!) turning the corner of realising that it really is all up to me, and the choices I make are the only ones that will make a difference. It's uncomfortable, because it means that I have to be accountable for any choices I make. But I'll get there. And I refuse to do the self-hatred thing when I make different choices (yes, dh bought me chocolate for v-day and I ate it. Yesterday at work I was presented with a lurid pink shortbread. I ate it. But I have also switched from carb heavy meals to salad and veggies and protein, and am generally walking away from the snacks...) It's a war, not a battle 
As I said up there somewhere, I like Anna Richardsons Body Blitz/ drop a dress size. It's two weeks. There are clearly defined rules. It gives me enough guidance without having to weigh anything, count anything, add up points, or whatever. Getting healthier should run alongside your life, not take it over completely!
Hope you stick around anyway! We all have multiple stones to lose, I think?