Mistress of the Gods by Rex Sumner

The king’s anger and jealousy at Susan’s success in running the kingdom caused her to flee. She hides her trail and makes for the hills, where she will study healing lore, banned by the church. This choice brings her to the notice of the Ancient Gods, who recruit her as a Shelagh na Gig, an ancient guardian of the goddess. Here she learns ancient lore, the Right Hand Path.
The king marches north to face the invader, and realises he faces a numerically superior foe, while his own army is riven with incompetent gentry and treacherous Churchmen, who do not want the king in power. His own professional regiments are upset at Susan’s disappearance, as is the Archbishop.

His daughter is growing in competence, and demanding her own squad, while noblemen are pushing for advancement. He faces an uncomfortable and unpopular decision, but is helped by the arrival of a new type of cavalry from the south – lancers. Hard drinking, hard fighting womanisers. And the general sent his daughter, the princess, off to be their liaison…

Immersing himself in battle, seeking to forget his love, Susan, the king finds his time is past. Betrayal is in the air, but the new style of warfare will take time to appreciate. The princess needs no time, and finds you don’t need to go to war to win at politics.
Historical fantasy, with a wealth of accuracy in the detail of crops and a 20,000 year old religion brought to life. Gritty, realistic, from a violent time. Figures from history may be woven into the story.