Based on what you have said, you should be able to claim your adult child as a qualifying relative dependent. Since you lived apart from your husband the entire last half of 2018, you are considered unmarried for tax filing purposes. These two things most likely allow you to file as Head of Household which is a much better filing status than Married Filing Separately which would be your only other option if you don't file Married Filing Jointly.
Here are the requirements for filing Head of Household:
1. You are not married or “considered unmarried” on the last day of the year. You are considered unmarried if you lived apart from your spouse the entire last 6 months of the year.
2. If "considered unmarried" you file a separate return from your spouse.
3. You paid more than half the cost of keeping up your home for the year.
4. You have a qualifying child (whether or not you claim the dependency exemption) or you claim a dependency exemption for a “qualifying person” who is related to you and who lived with you in the home for more than half the year. An exception is a parent does not have to live with you to be a qualifying person.
