Two things:
1) A new instance of the Main form is not the same as the currently displayed instance of the Main form, in the same way that a new car is not the same as your existing car. If you go for a test drive in a new car, and put your mobile phone in the glove box, would you expect to find it again by opening the glove box of your old car?
Instead, use events: The Form1 raises an event which is handled by it's "parent" Main form, and the Main form then creates and handles the new instance of Form2.
This explains how to set up the event:
Transferring information between two forms, Part 2: Child to Parent[
^]
2) Why, oh why, are you embedding forms in tab pages? Why not use a UserControl, since that is what they are designed for? It's simpler, cleaner, and you don't get a load of garbage with it...