How can i get the correct information to display in my listbox?

I have two Listboxes where you can move exercises from an available list of exercises to a list of selected exercises for a specific user. My current problem is that my selected list (aka RegimeItems) shows the exercises for all of the users.

I believe the problem lies in ChosenExercises(at the bottom of controller) i am also getting a null reference exception at:

var regimeIDs = model.SavedRequested.Split(',').Select(x => int.Parse(x));

Controller.cs

    [HttpGet]
            public ActionResult ExerciseIndex(int? id, UserExerciseViewModel vmodel)
            {
                User user = db.Users.Find(id);
                user.RegimeItems = ChosenExercises();
                UserExerciseViewModel model = new UserExerciseViewModel { AvailableExercises = GetAllExercises(), RequestedExercises = ChosenExercises(user, vmodel) };
                user.RegimeItems = model.RequestedExercises;
                return View(model);
            }

            [HttpPost]
            public ActionResult ExerciseIndex(UserExerciseViewModel model, string add, string remove, string send, int id)
            {
                User user = db.Users.Find(id);
                user.RegimeItems = model.RequestedExercises;;
                RestoreSavedState(model);
                if (!string.IsNullOrEmpty(add))
                    AddExercises(model, id);
                else if (!string.IsNullOrEmpty(remove))
                    RemoveExercises(model, id);        
                SaveState(model);
                return View(model);
            }

            void SaveState(UserExerciseViewModel model)
            {
                model.SavedRequested = string.Join(",", model.RequestedExercises.Select(p => p.RegimeItemID.ToString()).ToArray());
                model.AvailableExercises = GetAllExercises().ToList();
            }

            void RestoreSavedState(UserExerciseViewModel model)
            {
                model.RequestedExercises = new List<RegimeItem>();

                //get the previously stored items
                if (!string.IsNullOrEmpty(model.SavedRequested))
                {
                    string[] exIds = model.SavedRequested.Split(',');
                    var exercises = GetAllExercises().Where(p => exIds.Contains(p.ExerciseID.ToString()));
                    model.AvailableExercises.AddRange(exercises);
                }
            }

            private List<Exercise> GetAllExercises()
            {
                return db.Exercises.ToList();
            }

            private List<RegimeItem> ChosenExercises(User user, UserExerciseViewModel model)//RegimeItem regimeItem)
            {
var regimeIDs = model.SavedRequested.Split(',').Select(x => int.Parse(x));
            return db.RegimeItems.Where(e => regimeIDs.Contains(e.RegimeItemID)).ToList();
            //return db.Users.Where(r => r.RegimeItems = user.UserID);
            //return db.Users.Where(r => r.RegimeItems.RegimeItemID == user.UserID);
            }

Models

  public class User
    {
        public int UserID { get; set; }
        public ICollection<RegimeItem> RegimeItems { get; set; }
        public User()
        {
            this.RegimeItems = new List<RegimeItem>();
        } 
    }
    public class RegimeItem
    {
        public int RegimeItemID { get; set; }
        public Exercise RegimeExercise { get; set; }
    }

ViewModel

public class UserExerciseViewModel
{
    public List<Exercise> AvailableExercises { get; set; }
    public List<RegimeItem> RequestedExercises { get; set; }
    public int? SelectedExercise { get; set; }
    public int[] AvailableSelected { get; set; }
    public int[] RequestedSelected { get; set; }
    public string SavedRequested { get; set; }
}