General synonyms:
* Regretfully
* Sorrowfully
* Sadly
* Mournfully
* Regretting
* With remorse
More specific synonyms:
* Bitterly: Emphasizes a strong sense of pain and disappointment.
* Wistfully: Emphasizes a longing for something lost or unattainable.
* Penitently: Emphasizes a feeling of guilt and a desire for atonement.
* Dejectedly: Emphasizes a feeling of low spirits and discouragement.
The best synonym for "ruefully" will depend on the context of your writing and the specific emotion you want to express.