Forgot to include something on tax return

HI. What if you forget to include something on your tax return, but have already filed it..... I have incurred a capital gain due to the sale of a property, and see that I should have included some share sales, which incurred a loss. Would they have been set off against each other?

It would be best to resubmit. You can do this by navigating to the tax return history in eFiling and "request correction" and then resubmit completing all the details.