The High Priestess is a woman who is in charge of her own life - therefore her feelings are influenced by the fact she is not psychologically owned, directed or otherwise beholden to anyone (even if she is married with 10 kids!). I like Jenessa's word "self-possession" - which expresses exactly that quality of the High Priestess.
The High Priestess is also the gestator of the tarot. Ideas, children, thoughts grow in the depth of her being, in the dark, before they come out in the light of the Empress's garden. Her feelings therefore are protective and patient, turned towards making sure the gestation is serene and without trouble.
I think I can understand what Willowfox means by fluctuating feelings. The High Priestess is linked to the Moon - I would say her feelings are cyclical rather than fluctuating without rhyme or reason. The High Priestess can also represent a menstruating or premenstrual woman because of the link between the Moon and menstruation - with all the different feelings that time in a woman's month brings! A reversed High Priestess can show trouble with PMS for instance. I found that meditating with the High Priestess helped with pre-menstrual trouble - she is a healer in that sense!
The High Priestess has some strong connection with Mary, the Virgin - and through that connection, I see her feelings as compassionate and ardent. She is able to suffer with others, and feel their pain as her own. She surrounds all those who need love with her tender care.
The High Priestess has been linked for a long time to Isis (who is in turn related to the Virgin Mary). Many decks actually represent her as Isis. The love of Isis for her husband and brother Osiris is well-known. When he was killed by their brother Set and his remains scattered, she wept so much she flooded the countryside. As she wept, she collected the remains of her beloved and chanted spells - finally reviving him. They had one night of love together, during which Horus was conceived, and then Osiris left to take up his place as King of the Underworld, for even Isis could not defeat death entirely. Isis/the High Priestess is therefore the epitome of the passionate lover, who will weep their heart out when they lose the one they love, yet at the same time retain enough self-possession (that word again) to work on the situation. The High Priestess, more than any other arcanum, is capable of huge self-sacrifice for the one she loves, of unstinting devotion, and of using her skill and imagination in pursuit of her love.