My will was prepared and is ready to be executed. I forgot to include my middle name as the Testator. Does it have to be rewritten or can I just insert it myself?

If the Will was typed get it corrected before you sign. If that would be difficult or impossible based on how the document was prepared you could insert the initial and when you sign initial by hand where you inserted the initial. So if your name is John C. Doe and you insert the C. then initial next to the C. using JCD.

If it is clear from the Will who you are the lack of the middle initial would not invalidate the Will but best to correct if you can.