how to change values in a textbox without replacing the previous values

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C#

i have a form with a textbox and the text box gets dynamicaly loaded on the form load. i need to assign multiple values to the textbox without replacing the previous values when ever the form is loaded. for example: i have 2 items when i click on the first item the form gets loaded and i enter 45 in the textbox and then i click the second item the form again gets loaded and i enter 54 in the textbox and now when i go to the first item and load the textbox i want the value to be 45 and shouldn't be replaced with 54.. how do i do this??
i have an expander and several icons in it when i drag and drop an item from it to a canvas a pop up window comes asking me to assign a value for the item. on right clicking the item on the canvas another window comes up showing the value that has been entered in the previous pop up form. and if i drag an drop the same kind of icon again from the expander the same procedure happens but the value of the first icon gets replaced by the newly entered value


EDIT: From Comments:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;


namespace MotionControl.PropertiesForms
{
public partial class property : Form
{
instance inst;
Micropositionclass micro;

public property()
{
InitializeComponent();
micro = Micropositionclass.Instance;

inst = instance.Instance;
if(inst.firstGuidList.Contains(inst.a))
{
this.Text = "MPC1";

}

else if (inst.secondGuidList.Contains(inst.a))
{

this.Text = "MPC2";


}


else if (inst.thirdGuidList.Contains(inst.a))
{
this.Text = "MPC3";

}
else if (inst.fourthGuidList.Contains(inst.a))
{
this.Text = "MPC4";

//MPC.Default.Save();
}
else if (inst.fifthGuidList.Contains(inst.a))
{
this.Text = "MPC5";

//MPC.Default.Save();
}
else if (inst.sixthGuidList.Contains(inst.a))
{
this.Text = "MPC6";

//MPC.Default.Save();
}
else if (inst.seventhGuidList.Contains(inst.a))
{
this.Text ="MPC7";

//MPC.Default.Save();
}
else if (inst.eighthGuidList.Contains(inst.a))
{
this.Text = "MPC8";


//MPC.Default.Save();
}
else if (inst.ninthGuidList.Contains(inst.a))
{
this.Text = "MPC9";

//MPC.Default.Save();
}
else if (inst.tenthGuidList.Contains(inst.a))
{
this.Text = "MPC10";

//MPC.Default.Save();
}
switch (inst.tooltip)
{
case "Move":
this.Size= new Size(238,226);
Label lbl1 = new Label();
lbl1.Text = "X";
lbl1.Location = new Point(15, 35);
lbl1.Size = new Size(14, 13);
Controls.Add(lbl1);
Label lbl2 = new Label();
lbl2.Text = "Y";
lbl2.Location = new Point(15, 70);
lbl2.Size = new Size(14, 13);
Controls.Add(lbl2);
Label lbl3 = new Label();
lbl3.Text = "Z";
lbl3.Location = new Point(15, 105);
lbl3.Size = new Size(14, 13);
Controls.Add(lbl3);
TextBox tb1 = new TextBox();
tb1.Text = micro.movex.ToString();

tb1.Location = new Point(105, 35);
tb1.Size = new Size(100, 20);
Controls.Add(tb1);
TextBox tb2 = new TextBox();
tb2.Text = micro.movey.ToString();
tb2.Location = new Point(105, 70);
tb2.Size = new Size(100, 20);
Controls.Add(tb2);
TextBox tb3 = new TextBox();
tb3.Text = micro.movez.ToString();
tb3.Location = new Point(105, 105);
tb3.Size = new Size(100, 20);
Controls.Add(tb3);
break;
case "Speed":
this.Size = new Size(220, 162);
Label speed = new Label();
speed.Text = "Speed";
speed.Location = new Point(20, 40);