Please help in finding a book/possibly series title

[Kal]

Could I please get some help in finding the title of this fantasy book, possibly the first of a series, can't recall the title or any character names, however the details of it were:

 

A world of male sorcerers, immortal by way of when their power ran out, latching onto the soul/lifeforce of a human being somewhere in the world, though they didn't know who they were currently attached to/draining

 

A young female witch, wanting to become a sorcerer herself, no one in the world but the sorcerers knows what keeps them immortal, the more a witch uses her powers, the sooner she dies, so the girl had been keeping from using power.

 

She apprentices herself to a sorcerer in hopes of becoming one, he makes her do the opposite, use up all her power, not telling her why, until she uses the last of her lifeforce, manages to latch onto another person and become a sorcerer herself, the person being a prince of some powerful kingdom, the sorcerers pretend they don't know what's ailing him when he begins to fade/lose life as they don't want the secret known that a sorcerer is killing the heir.

 

There is also a witch queen out somewhere in the world who rules an island, who continues to look young herself for a hundred years or so, but not from using her own power, rather visiting sorcerers heal her body to a look of youth each time they visit, and she keeps news of all the sorcerers herself, kind of being a go between for them all gossipwise of everything going on with the sorcerers. she has some kind of token of each of the sorcerers in a chest.

This witch queen realises she only has a flicker of power/life left when trying to do a very minor spell, and will soon lose it all, begins to think bitter thoughts.

 

No one knows the young girl witch is a sorcerer except her teacher, so one random sorcerer provokes the girl into a minor battle atop a tower with gusting winds etc, thinking he's forcing a witch into using up their own lifeforce when she responds, she bests the sorcerer by him running out of the person he's currently draining's lifeforce, he falls and dies, having failed to attach to another before hitting the ground.

 

The girl witch/sorcerer discovers who she's draining, the prince, every time she thinks of the fact she's killing him she almost loses grip on life herself, until she forces herself to want it more than caring about him again, she has sex with the prince at least once.

 

At the end of the book the girl disappears/escapes after fulling draining the lifeforce of the prince, which shows she's managed to attach to another person.