MenuStrip not updating when editing it from another form (C#)

I've been having an issue that's been bugging me for the past 3-4 hours. Basically, I have a main form, that is an MDI parent. I have another form, that is an MDI child of that parent. I use the second form for logging in and after I've been logged in, I want to add new items to the menuStrip of that main form, but for some reason I can't. I tried debugging and I saw that the menuStrip's indexes are expanding correctly for the new items, but they are not being updated in the form. I tried displaying the main form again with this.Show. This worked! I got a new form displaying the new items in the menuStrip. However, I just want to update them and not create another form. I tried refreshing/updating, but nothing worked. :/ Any suggestions?

Thank you!

EDIT: Here is some more info:

this is the code from the 2nd form

if (successLogin==DialogResult.OK || successLogin==DialogResult.Cancel)
                {
                    Form1 main = new Form1(); //define main form
                    FullUser = textBox1.Text; //getting the username (will use it for the menuStrip items "Logged in as: "+ FullUser)
                    main.LogedIn(FullUser);//calls method

                    this.Close(); //closing the 2nd form

                }

that calls for the public method in the main form

    public void LogedIn(string user)
    {
        menuStrip1.Items.Add("Item here"); //adding the item to the menuStrip
    }

Edit 2: I put a timer which starts when opening the 2nd form and checks for a public boolean. That got it working, but still not the way I wanted it to.