I forgot to add a form to my return but I hadn't sent it out yet. Now that I amended it, why is the refund amount so different?

If you did not send in the original return, you shouldn't amend it, you should simply update, or correct, it. 

You can't file an amended return until after you file an original return. Filing both an original and amended return at the same time will cause a kerfuffle because they will cross one-another in the system. 

An amended return assumes the original return has processed and shows the difference, so if the original showed a refund, the 1040X assumes you already received that refund (or paid that tax). 

If possible, delete the amendment(1040X) and reopen the original return.  Add the additional form and file. 

First cancel the amended return.

1. Open your return if it’s not already open.
2. In the left menu select Cancel amend.
3. Select Yes, I want to cancel my amended return and Continue.

If you can't get into your return, click "Your tax return & documents" on the home page, then click "Add a State" to open the return. Cancel the 1040 (amendment) and add the additional tax form. Review the federal and state returns and file. 

It would make no sense to file the incorrect return.