Finding which customtextbox sends the message to customdatagridview

I have a customtextbox and a customdatagridview . the customtextbox on its OnKeyDown event behaving as tab and on its back key behaving as reverse tab respectively . Now on my customdatagridview's OnEnter, I want to analyze which customtextbox is sending the focus to customdatagridview so that I change my current cell accordingly , but I am confused as how to get it .

public partial class betterdatagridview : DataGridView
    {
    protected override void OnEnter(EventArgs e)
    {
        // datagridview has data 
        if (this.RowCount > 0)
        {

            if () // what to place here to check which customtextbox send me here 
            {
                this.CurrentCell = this.Rows[0].Cells[0]; // select the first cell in the first row
            }
            else if()
            {
                this.CurrentCell = this.Rows[RowCount - 1].Cells[0];//[0, this.RowCount - 1]; // select the last row's first cell
            }
         
            this.BeginEdit(true); // it must stay here 
            TextBox textBox = (TextBox)this.EditingControl; // Cast the EditingControl to TextBox
            textBox.SelectionStart = textBox.Text.Length; // select the textbox from last 
            
        }

and below is the customtextbox

  protected override void OnKeyDown(KeyEventArgs e)
    {
        if (!DisableFeatures)
        {
            if ((e.KeyCode == Keys.Enter || e.KeyCode == Keys.Down)&& this.Text !="")
            {
                e.Handled = true;
                e.SuppressKeyPress = true;
                TriggerEvent?.Invoke(Last, Name);
                if (!Last)
                {
                    SendKeys.Send("{TAB}");
                }
               
                return;
            }
            if ((e.KeyCode == Keys.Back || e.KeyCode == Keys.Up) && this.Text.Length == 0)
            {
                SendKeys.Send("+{TAB}");
            }
        }
        base.OnKeyDown(e);          
    }