I file 2019 taxes and got my return, but forgot to add my expenses for being an new inventor with a patent pending. I spent a lot of money, which i think i should have report it for a deduction.
You might want to consult with a patent attorney. In the meantime see this Nolo Press articel.
if you had a patent pending then it would seem you've done nothing to actively start your business like soliciting customers.
the IRS position
It might be best to say that a business "starts" when it first starts advertising and is ready to serve customers, as a business can start when it begins advertising, even though no sales have been made.
if business hasn't started all those expenses and any you already deducted would likely be non-deductible start-up expenses. this would require you to amend your return.