Mortgage interest you paid is reported to you every year on a Form 1098.  You must report a 2017 Form 1098 on a 2017 return, a 2018 1098 on a 2018 return, etc.

If you did not include a 1098 on your original return for a particular year, you must file an amended return for that year.  You cannot claim a Form 1098 on a different year's tax return.

Not sure why you thought you had to own the house for a whole year before you could enter anything about it on your 2017 tax return.  Find your 1098 from the lender for 2017 and amend the 2017 return.  You can enter mortgage interest, property taxes, loan origination fees and private mortgage insurance paid in 2017.