Forgetful, oblivious, unmindful are comparable when they mean losing or letting go from one’s mind something once known or learned.
Forgetful usually implies a propensity not to remember or a defective memory. Sometimes it implies a not keeping in mind something which should be remembered; it then connotes negligence or heedlessness rather than a poor memory.
Oblivious stresses forgetfulness, but it rarely suggests a poor memory. Rather, it suggests a failure to remember, either because one has been robbed of remembrance by conditions beyond one’s control or because one has deliberately put something out of one’s mind or because one has considered something too slight or trivial to note and remember it.
In some instances oblivious is employed without a clear connotation of forgetfulness, and in a sense close to unconscious, unaware, and insensible.
Oblivious also is sometimes used attributively and without a succeeding of or to in the sense of causing oblivion.
Unmindful is a close synonym of forgetful in the sense of not keeping in mind, but it may imply a deliberate consignment to oblivion as well as inattention, heedlessness, or negligence.