How can a woman preserve her honor and herself from falling into sin during her husband's prolonged absence, and are there any supplications to protect her from forbidden acts?

A woman protects herself in her husband's absence by seeking refuge in Allah, adhering to the rulings of the Sharia, and staying away from situations of suspicion and temptation. She should not allow a non-mahram man to enter upon her, nor mix with him, except out of necessity, while observing the lawful hijab and lowering her gaze. There is no specific supplication for this, but supplication in general is beneficial. Among the reasons by which a wife is protected is for the husband to fulfill his duty of qawamah (guardianship), and to close the doors to temptation. "Guard Allah, and He will guard you." It is also incumbent upon the husband not to prolong his absence from his wife without a valid excuse.