Depression or fatigue, which are indirect (or secondary) symptoms of MS, may play as large a part in the way that you feel sexually as does primary neurological damage. If such symptoms are treated successfully, then your sexual drive (often called your libido) may increase.
If the primary cause of your decreasing sexual drive lies in primary neurological damage, then this is harder to deal with directly. You and your partner could consider first sensual activity experiences, without you feeling the immediate pressure for sexual intercourse. Some parts of your body will probably be more sensitive than others. Ensure that you make time to enjoy the experiences with each other without feeling hurried or under pressure. As in other relationships where circumstances change, new, and possibly exciting and stimulating, patterns of mutual exploration may need to be learnt or re-learnt.