Hi Ana; I'm new to this posting also but might be able to help.

You probably need to strengthen your core muscles which support your body. The best way is to find a Yoga instructor (local MS Chapter can help) and work those muscles daily to keep them from letting your body down. A strong body core is essential. I've been through what you are experiencing and Yoga was the only thing that worked for me. Hope this helps you too.
Have a great day, John