You misunderstood something that you read, or mixed up two different rules.
You can file jointly with your late wife only for the year of her passing, 2018. For the two years after that (2019 and 2020), if you do not remarry, and if you have a dependent child, you can file as qualifying widower. If you do not have a dependent child, and you have not remarried, you have to file as single (or possibly head of household if you meet the requirements).
But since you did remarry, your only choices for filing status are married filing jointly with your new wife, or married filing separately. VolvoGirl has outlined some of the disadvantages of filing as married filing separately. Those disadvantages apply to both you and your wife, who would also have to file separately. Unless you have a very strong reason for filing that way, and have evaluated how much it will cost you, you and you wife should seriously consider filing jointly.
