If your 19 year old is a full-time student then you may be able to file Head of Household.
Am I Head of Household?
https://ttlc.intuit.com/questions/1894553-do-i-qualify-for-head-of-household
https://ttlc.intuit.com/questions/2900097-what-is-a-qualifying-person-for-head-of-household
If you qualify as Head of Household, when you enter your filing status (single or married filing separately) into MyInfo, and then enter your qualifying dependent, TurboTax will offer HOH as your filing status.
If under the age of 19 as of December 31, 2018, you can file an amended return for 2018 and prior years, claiming head of household.
Ditto for 2017.
What you describe qualifies you as Head of Household status.
However, your 19 year old daughter MUST be a full time student. More information here: https://turbotax.intuit.com/tax-tips/family/guide-to-filing-taxes-as-head-of-household/L4Nx6DYu9
Also, review the IRS information to be sure. https://www.irs.gov/help/ita/what-is-my-filing-status
Sorry for the loss of your wife.