If "bitterly" refers to intense emotion:
* Warmly: This emphasizes a positive and affectionate feeling.
* Gently: Suggests a soft and mild emotion.
* Sweetly: Implies a pleasant and loving emotion.
If "bitterly" refers to something harsh or severe:
* Mildly: Means something is not harsh or severe.
* Gently: Suggests something is done with care and delicacy.
* Kindly: Implies something is done with good intentions.
If "bitterly" refers to something unpleasant or painful:
* Pleasantly: Suggests something is enjoyable and enjoyable.
* Delighfully: Implies something is very enjoyable.
* Joyfully: Means something is done with happiness and delight.
Ultimately, the best opposite for "bitterly" will depend on the specific context in which it is used.